code: def vatu0(self,E,Lz,u0,R,retv2=False):
                                  
          v2= (2.*(E-actionAngleStaeckel.potentialStaeckel(u0,numpy.pi/2.,
                                                           self._pot,
                                                           self._delta))
               -Lz**2./R**2.)
          if retv2: return v2
          v2[(v2 < 0.)*(v2 > -10.**-7.)]= 0.
          return numpy.sqrt(v2)
docs: NAME:
             vatu0
          PURPOSE:
             calculate the velocity at u0
          INPUT:
             E - energy
             Lz - angular momentum
             u0 - u0
             R - radius corresponding to u0,pi/2.
             retv2= (False), if True return v^2
          OUTPUT:
             velocity
          HISTORY:
             2012-11-29 - Written - Bovy (IAS)