code: func (p *parser) undo(t *Token) {
  	// add this token to the list of previously read tokens
  	//      - initialise the undo list if required
  	if p._undo == nil {
  		p._undo = make([]*Token, 0, 1)
  	}
  	p._undo = append(p._undo, t)
  }
docs: // undo returns the given Token t to the parser input stream to be retrieved
  // again on a subsequent call to next.