code: def Rzderiv(self,R,Z,phi=0.,t=0.):
          
          try:
              return self._amp*self._Rzderiv(R,Z,phi=phi,t=t)
          except AttributeError: #pragma: no cover
              raise PotentialError("'_Rzderiv' function not implemented for this potential")
docs: NAME:
  
             Rzderiv
  
          PURPOSE:
  
             evaluate the mixed R,z derivative
  
          INPUT:
  
             R - Galactocentric radius (can be Quantity)
  
             Z - vertical height (can be Quantity)
  
             phi - Galactocentric azimuth (can be Quantity)
  
             t - time (can be Quantity)
  
          OUTPUT:
  
             d2phi/dz/dR
  
          HISTORY:
  
             2013-08-26 - Written - Bovy (IAS)