in the following detailed description of embodiments of the present disclosure , reference is made to the accompanying drawings in which like references indicate similar elements , and in which is shown by way of illustration specific embodiments in which the present disclosure may be practiced . these embodiments are described in sufficient detail to enable those skilled in the art to practice the present disclosure , and it is to be understood that other embodiments may be utilized and that logical , mechanical , electrical , functional , and other changes may be made without departing from the scope of the present disclosure . the following detailed description is , therefore , not to be taken in a limiting sense , and the scope of the present disclosure is defined only by the appended claims . as used in the present disclosure , the term “ or ” shall be understood to be defined as a logical disjunction and shall not indicate an exclusive disjunction unless expressly indicated as such or notated as “ xor .” as used herein , the term “ real time ” shall be understood to mean the instantaneous moment of an event or condition , or the instantaneous moment of an event or condition plus short period of elapsed time used to make relevant measurements , optional computations , etc ., and communicate the measurement , computation , or etc ., wherein the state of an event or condition being measured is substantially the same as that of the instantaneous moment irrespective of the elapsed time interval . used in this context “ substantially the same ” shall be understood to mean that the data for the event or condition remains useful for the purpose for which it is being gathered after the elapsed time period . as used herein , the term “ compressible member ” shall be understood to mean devices that cause a gas or fluid to be compressed when a gas or fluid is placed into a chamber where the compressible member is disposed . examples of compressible members include , for example , closed cell foams , elastomeric diaphragms , pistons , and secondary chambers charged with a fixed volume of gas , wherein when a gas or fluid is placed into the chamber in which the compressible member is disposed , the gas or fluid in the compressible member compresses . the present inventor has discovered a device and method for preventing inadvertent flow of flow materials from a pump . the device comprises a stopcock having a chamber built in that is charged with aliquots of flow material . the stopcock is moved from a first position that is used for charging ( filling ) the stopcock into at least a second position that is configured for dispensing the stored aliquots of flow material . one or more pressure sensors are disposed to measure , in real time , the pressure of the chambers holding the flow materials , thereby deriving the volume of flow material dispensed from the stopcock device . because flow volumes and the elapsed time are also know , the flow rate of the dispensed flow material may be calculated , according to embodiments . temperature sensors may similarly be disposed to improve the accuracy of the calculations used to measure the flow volume or rate . according to embodiments , stopcock device 100 is illustrated generally in fig1 . stopcock device 100 comprises first chamber 110 , stopcock 150 , and conduits placing first chamber 110 and stopcock 150 into fluid communication and allowing for dispensing of the flow material . stopcock is connected to positioning device 158 , which positions stopcock 150 into one of a plurality of positions for at least charging chambers within stopcock with flow material and into position for dispensing flow material . within stopcock 150 is at least one second chamber 152 and corresponding second chamber conduit 154 . according to embodiments , first pressure sensor 120 and second pressure sensor 122 are disposed at locations in stopcock device 100 that allow for the accurate measurement of the pressure of first chamber 110 and second chamber 152 , thereby allowing calculation of flow volume and flow rates of flow materials based on change of pressure calculations , as described in detail below . a computer performs the relevant calculations . the computer comprises at least a timing device for measuring elapsed time , which may comprise a clock or a timer , for example ; devices to receive input from the pressure sensors , temperature sensors , and users ; and a processor for performing the calculations disclosed herein . first chamber 110 is a chamber of known volume . according to embodiments , first chamber 110 volume comprises first chamber 110 and first chamber / stopcock conduit 132 . according to embodiments , first pressure sensor 120 is disposed such that it is in at least gas communication with first chamber 110 or first chamber / stopcock conduit 132 , depending on the components of the present disclosure comprising first chamber 110 . fill conduit 130 comprises a conduit for filling first chamber 110 with a flow material . conduits may comprise any conventional device used to sealably transport gases or flow materials , for example pipes , tubes , or other conduits defined by a sealed body terminating in one or more open ends through which a flow material or gas enters and exits the interior of the sealed body . according to embodiments , first chamber 110 comprises a chamber having a quantity of gas contained in a compressible member . filling of first chamber 110 does not displace gas ; in other words , the net number of gas molecules remains constant as flow material fills first chamber 110 , thereby pressurizing the gas as the volume of the gas decreases . this may be accomplished by installing a one way valve , such as a check valve , as part of fill conduit 130 . the exact amount of gas does not need to be known , provided the differential pressure can be measured and the total volume of first chamber is known . according to embodiments , gas in first chamber 110 may be contained in a secondary chamber ( i . e ., the compressible member ). the secondary chamber comprises , according to various embodiments , a gas “ pillow ” formed from a flexible diaphragm that compresses when flow material fills first chamber 110 ; a movable , sealed divider ( e . g ., a piston ) that compresses when flow material fills first chamber 110 , etc . according to embodiments , gas may also be contained in a closed cell foam that occupies substantially all or part of first chamber 110 . in each of these embodiments , the compressed gas will exert pressure on the flow material whereby flow material will flow into second chamber 152 , as described below . according to embodiments , in addition to , or instead of pressure , gravity or a pump such as a lead screw may be used to move flow material from first chamber to second chamber 152 , as described below . according to embodiments and as illustrated in fig1 , first chamber 110 has disposed therein compressible member 112 . according to embodiments , compressible member 112 comprises closed cell foam . compressible member 112 may also comprise rubber or another compressible material that effects a differential gas pressure in first chamber 110 when first chamber 110 is charged with flow material verses when first chamber 110 is not charged with flow material . according to embodiments , compressible member 112 comprises a pocket of air contained within a compressible bag or “ pillow .” according to embodiments , compressible member is disposed in first chamber 110 to ensure sufficient pressure to move the last amount of flow material from first chamber 110 to second chamber 152 . according to other embodiments , compressible member is not used , as described above . indeed , as flow material is put into first chamber 110 , the flow material is under pressure or delivered through a one - way valve . thus , flow material will naturally flow from first chamber 110 to second chamber 152 due to the pressure exerted by the gas that is pressurized as first chamber 110 is filled . because the gas or closed cell foam will tend to equalize as flow material moves out of first chamber , the last amount of flow material will be difficult to remove . removal of this last amount of flow material may be accomplished by placing the gas in first chamber 110 under pressure initially , according to embodiments . according to other embodiments , closed cell foam will mechanically tend to remove this last amount of flow material in much the same way as the foam relaxes into substantially the entire chamber as the flow material flows out . finally , gravity or a pumping mechanism may be used to remove the last amount of flow material . according to embodiments , stopcock device 100 is disposed along a flow path to prevent flow of a flow material except in pre - determined configurations . stopcocks are well known and understood by artisans , and include any generic two - or three - way valves , for example . according to embodiments , stopcock 150 comprises second chamber 152 , which is a cavity disposed within stopcock 150 and second chamber conduit 154 , which provides fluid communication between second chamber 152 and other components of stopcock device 100 , as detailed herein . according to embodiments , second chamber conduit 154 comprises a plurality of conduits or chambers , whereby the fluid / gas communication features of the present disclosure connect with the different conduits or chambers . according to other embodiments , second chamber conduit 154 is a single conduit wherein each connecting conduit is situated to articulate with second chamber conduit 154 in substantially the same location relative to stopcock 150 depending on the position of stopcock 150 . within second chamber 152 , the amount of gas is constant like with first chamber 110 , and need not be known . the same devices may be used in second chamber with respect to the gas compressible member or secondary chambers may be configured with one or more one - way valves for the filling and dispensing of flow material to prevent back flow , as would be known and understood by artisans . for example , and according to embodiments , second chamber 152 likewise has disposed therein compressible member , such as a closed cell foam . like compressible member 112 , compressible member compresses when charged with flow material , thereby creating a pressure differential in second chamber 152 when second chamber 152 is charged with flow material versus when second chamber 152 is not charged with flow material . according to other embodiments and as shown in fig2 , second chamber 152 comprises flow material reservoir 152 a and a pressurized reservoir 152 b separated by a flexible or movable separator , such as an elastomeric membrane or piston . as flow material reservoir is charged , the membrane or piston stretches or moves , thereby compressing gas in the pressurized reservoir . the same gas in pressurized reservoir is used as the gas from which second pressure sensor 122 takes measurements . second pressure sensor 122 is disposed to be in gaseous communication with second chamber 152 . for example , second pressure sensor 122 is disposed immediately adjacent to second chamber 152 , as shown in fig1 . according to alternative embodiments , second pressure sensor 122 is separated from second chamber 152 by a pressure sensing conduit . according to embodiments , dispensing conduit 134 serves as a conduit from second chamber 152 to a target , for example a patient , where the flow material is intended . according to embodiments , flow regulator 160 may be disposed to modulate flow rate . flow regulator 160 comprises flow restrictors , for example . according to embodiments where it is used , a pressure sensing conduit serves as a conduit between second pressure sensor 122 and second chamber 152 when stopcock 150 is positioned so as to be in gaseous communication with pressure sensing conduit . according to embodiments , pressure sensing conduit comprises a small cavity having a pressure sensor ; according to other embodiments , pressure sensing conduit comprises a tube , pipe , or other conduit with second pressure sensor 122 disposed somewhere therein to measure the pressure . in all cases , the total volume of second chamber 152 , second chamber conduit 154 , and pressure sensing conduit is a known volume . positioning device 158 connects to stopcock 150 and effects repositioning of stopcock 150 . according to embodiments , positioning device 158 is a motor that rotates stopcock 150 . according to embodiments , positioning device 158 may also be a device that moves a slideable stopcock back and forth . according to embodiments , stopcock 150 occupies one of three positions : a fill position ( fig3 ) where second chamber 152 is in fluid communication with first chamber 110 , but not in communication with dispensing conduit 134 ; a closed position ( fig4 ) where second chamber 152 it is not in fluid communication with either first chamber 110 or dispensing conduit 134 ; and a dispense position ( fig5 ) where second chamber 152 is in fluid communication with dispensing conduit 134 , but not first chamber 110 and in which second pressure sensor 122 is in gaseous communication with second chamber 152 . according to embodiments and as illustrated in fig3 - 5 , when stopcock 150 is in a fill position ( fig3 ), an aliquot of flow material is moved from first chamber 110 into second chamber 152 , wherein second chamber 152 is filled with a flow material . once filled , flow material is ready to be dispensed to a target , according to embodiments . after second chamber 152 is filled with an aliquot of flow material , stopcock 150 is positioned in a closed position ( fig4 ) wherein second chamber 152 is sealed . according to embodiments , the pressure is measured in first chamber 110 or second chamber 152 to determine the volume of the aliquot transferred from first chamber 110 to second chamber 152 . artisans will note that pressure cannot be sensed for first chamber while first chamber 110 and second chamber 152 are in fluid or gas communication . when stopcock 150 is positioned in a closed or dispensing position ( fig4 and 5 ), second chamber 152 is in gaseous communication with second pressure sensor 122 , but not with first chamber 110 . in these positions , the pressure of the gas in second chamber 152 is sensed with second pressure sensor 122 . the total volume of second chamber 152 is known , thereby allowing the amount of flow material dispensed or remaining in second chamber 152 to be calculated based on the change in pressure of second chamber 152 . according to embodiments , after filling second chamber 152 with an aliquot of flow material or after dispensing flow material , stopcock 150 is positioned in the closed position , the pressure is measured , and the flow material volume calculated ( because the total volume of second chamber 152 when stopcock 150 is in the pressure sensing position is known ). to dispense flow material , stopcock 150 is positioned in its dispense position ( fig5 ) for a period of time . according to embodiments , stopcock 150 is then rotated to the closed position ( fig4 ) for a period of time that is used to measure the pressure . according to embodiments , pressure measurements are taken in the dispense position ( fig5 ) to determine the flow rate in about real time without changing the position of stopcock 150 . because the volume of the aliquot of flow material that filled second chamber 152 is calculated from the pressure in first chamber 110 or second chamber 152 , and because the total volume and initial values for the pressure of second chamber 152 are known , according to embodiments , stopcock need not be positioned in the closed position prior to dispensing the flow material , according to embodiments . according to alternate embodiments , stopcock 150 occupies one of four positions : a fill position ( fig3 ) wherein second chamber 152 is in fluid communication with first chamber 110 , but not in communication with dispensing conduit 134 ; a closed position ( fig4 ) where second chamber 152 is not in fluid communication with either first chamber 110 or dispensing conduit 134 ; a pressure sensing position , wherein second chamber 152 is in gaseous communication with second pressure sensor 122 and the pressure of second chamber may be measured ; and a dispense position where second chamber 152 is in fluid communication with dispensing conduit 134 , but not first chamber 110 . according to embodiments and as illustrated in fig6 , a method is disclosed for effecting flow of flow material from a source and through stopcock device 100 . during the fill operation , according to embodiments , stopcock 150 is initially positioned into the closed position ( fig4 ), wherein it is not in fluid communication with either first chamber 110 or second chamber 152 . in this position , an initial pressure measurement of first chamber 110 is made in operation 702 . according to alternate embodiments , no initialization is necessary as the system will be in a known state prior to filling ( for example , known volume of flow material in first chamber 110 and known initial volume of first chamber 110 ). in operation 704 , first chamber 110 is filled with flow material to a desired level and a second pressure measurement is taken in operation 706 , from which the total amount of flow material in first chamber 110 is calculated . after charging first chamber 110 with flow material , fill conduit 130 is closed and substantially sealed . according to embodiments , the filling of first chamber 110 may be omitted where the device comprises a disposable that is discarded when the flow material in first chamber is spent . in operation 704 , according to embodiments , flow material enters through fill conduit 130 and into first chamber 110 . flow of flow material into first chamber 110 via fill conduit 130 may be effected from any conventional pump , including specialized infusion pumps , for example those disclosed in u . s . pat . nos . 7 , 008 , 403 ; 7 , 341 , 581 ; and 7 , 374 , 556 , which are hereby incorporated by reference in their entirety . as flow material enters first chamber 110 , compressible member 112 or gas is compressed . thus , compressible member 112 or the gas stores the energy that will later be used to effect movement of an aliquot of flow material from first chamber 110 to second chamber 152 . after first chamber 110 is filled , flow material is ready to be dispensed from first chamber 110 to second chamber 152 . prior to filling second chamber 152 with and aliquot of flow material , an initial pressure measurement of second chamber 152 is taken , according to embodiments in operation 708 . stopcock 150 is positioned into the fill position in operation 710 , whereby flow material flows from first chamber 110 , through first chamber / stopcock conduit 132 and second chamber conduit 154 , and into second chamber 152 . as flow material enters into first chamber 110 , it compresses compressible member , according to various embodiments . according to embodiments , only a small aliquot of flow material moves from first chamber 110 into second chamber 152 as second chamber 152 charges . for example , first chamber 110 may have a volume of 3 ml and second chamber 152 may have a volume of 0 . 3 ml . the small aliquot of flow material transferred is the max amount of flow material that can be inadvertently delivered in the event of an error . after second chamber 152 is charged with flow material , the pressure of second chamber 152 is measured in operation 712 . the volume of the aliquot transferred to second chamber may be calculated either from the change in pressure from first chamber 110 or by measuring the change in pressure in second chamber 152 . for example , first pressure sensor 120 again measures the pressure of first chamber 110 . because flow material has been removed from first chamber 110 into second chamber 152 , the volume of compressible member 112 or the gas is increased , thereby reducing the pressure . thus , because the total volume of first chamber 110 is known , the amount of flow material transferred to second chamber 152 may be calculated , as shown below . similarly , the volume of the aliquot of flow material in second chamber 152 may be calculated from the difference in pressure in second chamber measured before the aliquot of flow material is transferred and after the aliquot of flow material is transferred to second chamber 152 . according to embodiments , both calculations may be used and an average value taken . once the volume of flow material in second chamber 152 is known , according to embodiments , stopcock 150 is positioned in its dispense position in operation 714 . because second chamber 152 also has disposed therein compressible member 112 or gas that is pressurized , flow material in second chamber 152 is dispensed due to the pressure exerted on it by compressible member 112 or the gas . according to embodiments , dispensing conduit 134 has disposed therein flow regulator 160 , which is , for example , a flow restrictor or clamping device designed to regulate the flow rate of flow material . second pressure sensor 122 , according to embodiments , measures the change in pressure as flow material is dispensed in operation 716 . according to embodiments in which stopcock 150 has a pressure sensing position , stopcock 150 is alternated between a dispensing position ( fig5 ) and a pressure sensing position to determine the volume of flow material that has been dispensed . according to embodiments in which stopcock does not have a pressure sensing position , the pressure changes are sensed in real time in the dispense position ( fig5 ). thus , the pressure change of the gas in second chamber 152 is gradual and predictable , allowing for a flow rate to be calculated . because the volume of second chamber 152 is known , the rate of flow may be calculated , as shown below . according to embodiments , the aliquot of flow material in second chamber 152 is small enough that determination of the flow from second chamber 152 is not required . in other words , because the aliquot size is so small , a flow rate with an acceptable level of error may be determined from the number of aliquots delivered over a period of time . according to embodiments , gas from first chamber 110 and second chamber 152 is not dispensed with the flow material . thus , the number of gas molecules in each respective chamber remains constant , as is required for the exemplary equations below to be true . artisans will readily appreciate that these exemplary equations illustrate the principles by which the volume of flow material dispensed is calculated . according to embodiments , stopcock device 100 is configured as an accessory to other pump devices whereby sterility of the flow material is maintained , but volumes of flow material are determined in about real time accurately . for example , the present disclosure is provided as an accessory to infusion pumps . the pumps may be conventional or nonconventional pumping devices . specialized pumps may also be used , including those with two , three , or more chambers , for example as disclosed in u . s . pat . nos . 7 , 008 , 403 ; 7 , 341 , 581 ; and 7 , 374 , 556 , and u . s . utility patent application ser . nos . 11 / 744 , 819 filed may 4 , 2007 and 12 / 020 , 498 , filed jan . 25 , 2008 ( the contents of each above listed patent and patent application are incorporated by reference ). indeed , the devices of the present disclosure may be provided as accessories for pumps that are able to measure flow rate in about real time . the devices of the present disclosure are also useful as safety devices for any pump , whereby upon an error state the maximum flow material that can be delivered to a patient upon a given error state is the small volume contained in second chamber 152 at the time of the error . according to embodiments and as illustrated in fig7 , stopcock device 100 also comprises computer 810 for controlling and performing the functions disclosed herein . computer may be any computer that is capable of being configured to receive input from users 822 or pressure sensors or temperature sensors 824 . computer also calculates flow volumes and flow rates 832 , checks for and detects error states 836 , and repositions stopcock 834 , and output audiovisual content 836 . according to embodiments , the ideal gas law is used to calculate the dispensed volumes of flow material . generally , the ideal gas law is expressed as : where p is the pressure of the gas in the chamber , v is the volume of the chamber , t is the temperature of the chamber , n is the number of moles of gas , and r is the universal gas constant . according to embodiments , first chamber 110 is of known total volume , containing a volume of gas . similarly , second chamber 152 is of known total volume , containing a volume of gas . as disclosed earlier , first and second chambers 110 , 152 may not have gas according to some embodiments , but other mechanical devices such as closed cell foam having therein a known volume of gas and a known compression profile , whereby the pressure readings of first chamber 110 are useful for the purposes of calculating a volume of flow material held in first chamber 110 . the following exemplary calculations may be adapted for such embodiments without undue experimentation . according to embodiments , because the total volume of first chamber 110 is known , the volume of flow material filling first chamber 110 , as well as the volume of aliquots of flow material dispensed into second chamber 152 may be calculated . likewise , because the total volume of second chamber 152 is known , the volume of flow material dispensed out of stopcock device 100 may be calculated . the exemplary calculations illustrate an embodiment whereby the volume of flow material dispensed is calculated . assuming constant temperature , the initial pressure of first chamber 110 is known or measured using first pressure sensor 120 . additionally , the total volume of first chamber 110 ( v 1 ) and second chamber 152 ( v 2 ) are known . accordingly , an initial volume of flow material is placed in first chamber 110 , thereby increasing the pressure of first chamber 110 . the amount of flow material in first chamber 110 ( v flowmaterial ) is calculated : where the volumes here measuring the initial and final gas volume , not flow material volume . if first chamber 110 is empty , then v 1initial = v 1 . otherwise , the computer will keep track of the initial volume of first chamber 110 for each successive aliquot . according to embodiments , where closed cell foam is placed in first chamber 110 , the calculation for pressure will account for the physical characteristics imparted by compression of the foam , as well . after filling first chamber 110 with a flow material and calculating the volume of flow material that is in first chamber 110 , fill conduit 130 is sealed . flow material may then flow from first chamber 110 to second chamber 152 . according to embodiments , stopcock 150 is rotated whereby it is in its fill position , as illustrated in fig3 . the pressure within first chamber 110 causes flow material to move from first chamber 110 to second chamber 152 , according to embodiments . according to other embodiments , gravity or mechanical devices can effect flow material movement from first chamber 110 to second chamber 152 . the volume ( v aliquot ) of flow material that fills second chamber is calculated : after second chamber is charged with an aliquot of flow material , stopcock is changed from its fill position ( illustrated in fig3 ) to another position , for dispensing , pressure measurement , or closed for accurate measurement of the pressure in first chamber 110 . to determine an amount of flow material dispensed at a given point , according to embodiments , similar calculation are performed for second chamber 152 to determine the amount of flow material dispensed . the gas volume of second chamber 152 after having receiving an aliquot is first determined : when stopcock 150 is positioned in its dispense position , flow material is dispensed from stopcock 150 by virtue of the pressure within second chamber 152 , gravity , or other mechanical forces , according to embodiments . accordingly , as flow material is dispensed out of second chamber 152 , the volume dispensed is calculated . the dispensed flow material volume is determined exactly opposite of the method by which the volume filled into first chamber 110 is calculated , according to embodiments . for example : according to embodiments , stopcock 150 is alternated between its dispense position and its pressure sensing position to make pressure measurements . according to other embodiments , stopcock 150 remains in its dispense position and the pressure of second chamber 152 is periodically assayed to determine the volume of flow material dispensed . a shortened version of the equations follows , where it isn &# 39 ; t necessary to determine the gas volume in second chamber 152 . v dispensed = v 2 ⁢ filled - v 2 ⁢ dispensed v dispensed = v 2 ⁢ filled - p 2 ⁢ filled ⁢ v 2 ⁢ filled p 2 ⁢ dispensed v dispensed = p 2 ⁢ dispensed ⁢ v 2 ⁢ filled p 2 ⁢ dispensed - p 2 ⁢ filled ⁢ v 2 ⁢ filled p 2 ⁢ dispensed v dispensed = v 2 ⁢ filled ⁡ ( p 2 ⁢ dispensed - p 2 ⁢ filled ) p 2 ⁢ dispensed v dispensed = v 2 ⁢ filled p 2 ⁢ dispensed ⁢ δ ⁢ ⁢ p 2 v dispensed = v 2 - v aliqout p 2 ⁢ dispensed ⁢ δ ⁢ ⁢ p 2 . according to embodiments , temperature sensors may be disposed in substantially the same locations as each pressure sensor to improve the accuracy of the calculations . the application of temperature into the exemplary calculation shown below are will within the skill and understanding of a person of ordinary skill in the art . according to embodiments , the above equations are adaptable to a single pressure sensor 122 in second chamber 152 . such a configuration is possible when knowing the initial volume in first chamber 110 is not necessary or desirable , or when the volume of first chamber 110 is known , for example when the device disclosed herein is a disposable . according , only the volume of each aliquot of flow material is calculated by measuring the pressure differentials in the second chamber 152 . for example , the volume of each aliquot ( p 2filled ) may be calculated independent of any calculations related to first chamber 110 : knowing the p 2filled value allows for the calculation of the dispensed volume as disclosed above . according to embodiments , because stopcock 150 eliminates direct fluid communication between the pump and the target of the flow material , it provides a safety mechanism for the pump . if a malfunction of the pump or stopcock device 100 occurs , the maximum unintended amount of flow material that can possibly be dispensed to the target is the small aliquot in second chamber 152 . in certain applications , for example in the delivery of insulin , these types of safety mechanisms are important for preventing unintended delivery of insulin due to malfunction . while the apparatus and method have been described in terms of what are presently considered to be the most practical and preferred embodiments , it is to be understood that the disclosure need not be limited to the disclosed embodiments . it is intended to cover various modifications and similar arrangements included within the spirit and scope of the claims , the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures . the present disclosure includes any and all embodiments of the following claims .
a dispenser using a stopcock valve has a first and second chamber . the first chamber has a flow material and is pressurized with air . the first chamber is connected to a stopcock valve that holds a second chamber which has a pressure sensor when the stopcock valve is rotated to the proper orientation . the stopcock valve is then rotated to a dispensing position where the flow material is dispensed under pressure to an outlet . by using the air pressure sensor and by taking into consideration the volume of the second chamber , the amount of flow of material can be dosed out consistently .