code: def cat(self, out_ndx=None):
          
          if out_ndx is None:
              out_ndx = self.output
          self.make_ndx(o=out_ndx, input=['q'])
          return out_ndx
docs: Concatenate input index files.
  
          Generate a new index file that contains the default Gromacs index
          groups (if a structure file was defined) and all index groups from the
          input index files.
  
          :Arguments:
             out_ndx : filename
                Name of the output index file; if ``None`` then use the default
                provided to the constructore. [``None``].