code: func RegisterManifestSchema(mediaType string, u UnmarshalFunc) error {
  	if _, ok := mappings[mediaType]; ok {
  		return fmt.Errorf("manifest media type registration would overwrite existing: %s", mediaType)
  	}
  	mappings[mediaType] = u
  	return nil
  }
docs: // RegisterManifestSchema registers an UnmarshalFunc for a given schema type.  This
  // should be called from specific