code: func (t *tree) peek() item {
  	if t.peekCount > 0 {
  		return t.token[t.peekCount-1]
  	}
  	t.peekCount = 1
  	t.token[0] = t.lex.nextItem()
  	return t.token[0]
  }
docs: // peek returns but does not consume the next token.