spec.go
func AddLoader(predicate DocMatcher, load DocLoader) {
	prev := loaders
	loaders = &loader{
		Match: predicate,
		Fn:    load,
		Next:  prev,
	}
	spec.PathLoader = loaders.Fn
}
func AddLoader(predicate DocMatcher, load DocLoader) {
	prev := loaders
	loaders = &loader{
		Match: predicate,
		Fn:    load,
		Next:  prev,
	}
	spec.PathLoader = loaders.Fn
}
// AddLoader for a document