promql/parse.go
func (p *parser) peek() item {
	if p.peekCount > 0 {
		return p.token[p.peekCount-1]
	}
	p.peekCount = 1

	t := p.lex.nextItem()
	// Skip comments.
	for t.typ == ItemComment {
		t = p.lex.nextItem()
	}
	p.token[0] = t
	return p.token[0]
}
func (p *parser) peek() item {
	if p.peekCount > 0 {
		return p.token[p.peekCount-1]
	}
	p.peekCount = 1

	t := p.lex.nextItem()
	// Skip comments.
	for t.typ == ItemComment {
		t = p.lex.nextItem()
	}
	p.token[0] = t
	return p.token[0]
}
// peek returns but does not consume the next token.