code: def next(self, vRef):
          
  
          num = _C.VSgetid(self._hdf_inst._id, vRef)
          _checkErr('next', num, 'cannot get next vdata')
          return num
docs: Get the reference number of the vdata following a given
          vdata.
  
          Args::
  
            vRef   Reference number of the vdata preceding the one
                   we require. Set to -1 to get the first vdata in
                   the HDF file. Knowing its reference number,
                   the vdata can then be opened (attached) by passing this
                   reference number to the attach() method.
  
          Returns::
  
            Reference number of the vdata following the one given
            by argument vref
  
          An exception is raised if no vdata follows the one given by vRef.
  
          C library equivalent : VSgetid