// the value returned by Lookup is unspecified.
func (tag StructTag) Lookup(key string) (value string, ok bool) {
	// When modifying this code, also update the validateStructTag code
	// in cmd/vet/structtag.go.

	for tag != "" {
		// Skip leading space.
		i := 0
		for i < len(tag) && tag[i] == ' ' {
			i++
		}
		tag = tag[i:]
		if tag == "" {