code: def update(self):
          
          pars = self.subpars.pars
          self(int(round(pars.control.lag)))
          pars.model.sequences.states.qjoints.shape = self+1
docs: Determines in how many segments the whole reach needs to be
          divided to approximate the desired lag time via integer rounding.
          Adjusts the shape of sequence |QJoints| additionally.
  
          Required control parameters:
            |Lag|
  
          Calculated derived parameters:
            |NmbSegments|
  
          Prepared state sequence:
            |QJoints|
  
          Examples:
  
              Define a lag time of 1.4 days and a simulation step size of 12
              hours:
  
              >>> from hydpy.models.hstream import *
              >>> parameterstep('1d')
              >>> simulationstep('12h')
              >>> lag(1.4)
  
              Then the actual lag value for the simulation step size is 2.8
  
              >>> lag
              lag(1.4)
              >>> lag.value
              2.8
  
              Through rounding the number of segments is determined:
  
              >>> derived.nmbsegments.update()
              >>> derived.nmbsegments
              nmbsegments(3)
  
              The number of joints is always the number of segments plus one:
  
              >>> states.qjoints.shape
              (4,)