code: def radialvelocity(self, rf='', v0='0m/s', off=None):
          
          loc = {'type': "radialvelocity",
                 'refer': rf,
                 'm0': dq.quantity(v0)}
          if is_measure(off):
              if not off['type'] == "radialvelocity":
                  raise TypeError('Illegal offset type specified.')
              loc["offset"] = off
          return self.measure(loc, rf)
docs: Defines a radialvelocity measure. It has to specify a reference
          code, radialvelocity quantity value (see introduction for the action
          on a scalar quantity with either a vector or scalar value, and when
          a vector of quantities is given), and optionally it can specify an
          offset, which in itself has to be a radialvelocity.
  
  
          :param rf: reference code string; Allowable reference
                     codes are: *LSRK LSRD BARY GEO TOPO GALACTO*
                     Note that additional ones may become available. Check with::
  
                         dm.list_codes(dm.radialvelocity())
  
          :param v0: longitude or x as quantity or string
          :param off: an optional offset measure of same type