promql/lex.go
func (l *lexer) emit(t ItemType) {
	l.items <- item{t, l.start, l.input[l.start:l.pos]}
	l.start = l.pos
}
func (l *lexer) emit(t ItemType) {
	l.items <- item{t, l.start, l.input[l.start:l.pos]}
	l.start = l.pos
}
// emit passes an item back to the client.