code: func (m *tree) String() string {
  	keys := m.Keys()
  	buf := bytes.NewBufferString("{")
  	for _, key := range keys {
  		val, _ := m.Lookup(key)
  		fmt.Fprintf(buf, "%s: %s, ", key, val)
  	}
  	fmt.Fprintf(buf, "}\n")
  	return buf.String()
  }
docs: // make it easier to display maps for debugging