code: def vcirc(self,R,phi=None):
          
          return nu.sqrt(R*-self.Rforce(R,phi=phi,use_physical=False))
docs: NAME:
          
              vcirc
          
          PURPOSE:
          
              calculate the circular velocity at R in potential Pot
  
          INPUT:
          
              Pot - Potential instance or list of such instances
          
              R - Galactocentric radius (can be Quantity)
          
              phi= (None) azimuth to use for non-axisymmetric potentials
  
          OUTPUT:
          
              circular rotation velocity
          
          HISTORY:
          
              2011-10-09 - Written - Bovy (IAS)
          
              2016-06-15 - Added phi= keyword for non-axisymmetric potential - Bovy (UofT)