code: func (s *MapSchema) MarshalJSON() ([]byte, error) {
  	return json.Marshal(struct {
  		Type   string `json:"type,omitempty"`
  		Values Schema `json:"values,omitempty"`
  	}{
  		Type:   "map",
  		Values: s.Values,
  	})
  }
docs: // MarshalJSON serializes the given schema as JSON.