the collector in `visitor_test` would be useful as a public object about proto HOT 4 CLOSED

type Filter func(v Visitee)

func WithService(apply func(*Service)) Filter {
	return func(v Visitee) {
		if s, ok := v.(*Service); ok {
			apply(s)
		}
	}
}

func Walk(proto *Proto, filters ...Filter) {
	for _, eachElement := range proto.Elements {
		for _, eachFilter := range filters {
			eachFilter(eachElement)
		}
	}
}

proto.Walk(yourDefinition, WithService(handleService), WithMessage(handleMessage))