code: def FRStaeckel(u,v,pot,delta): #pragma: no cover because unused
      
      R,z= bovy_coords.uv_to_Rz(u,v,delta=delta)
      return _evaluateRforces(pot,R,z)
docs: NAME:
         FRStaeckel
      PURPOSE:
         return the radial force
      INPUT:
         u - confocal u
         v - confocal v
         pot - potential
         delta - focus
      OUTPUT:
         FR(u,v)
      HISTORY:
         2012-11-30 - Written - Bovy (IAS)