code: def _force_xyz(self,x,y,z,i):
          
          return -4.*numpy.pi*self._b*self._c\
              *_forceInt(x,y,z,
                         lambda m: self._mdens(m),
                         self._b2,self._c2,i,glx=self._glx,glw=self._glw)
docs: Evaluation of the i-th force component as a function of (x,y,z)