code: def vx(self,*args,**kwargs):
          
          thiso= self(*args,**kwargs)
          if not len(thiso.shape) == 2: thiso= thiso.reshape((thiso.shape[0],1))
          if len(thiso[:,0]) == 2:
              return thiso[1,:]
          if len(thiso[:,0]) != 4 and len(thiso[:,0]) != 6:
              raise AttributeError("orbit must track azimuth to use vx()")
          elif len(thiso[:,0]) == 4:
              theta= thiso[3,:]
          else:
              theta= thiso[5,:]
          return thiso[1,:]*nu.cos(theta)-thiso[2,:]*nu.sin(theta)
docs: NAME:
             vx
          PURPOSE:
             return x velocity at time t
          INPUT:
             t - (optional) time at which to get the velocity
             vo= (Object-wide default) physical scale for velocities to use to convert
             use_physical= use to override Object-wide default for using a physical scale for output
          OUTPUT:
             vx(t)
          HISTORY:
             2010-11-30 - Written - Bovy (NYU)