code: func (t *tree) boolAttr(attrs map[string]string, key string, defaultValue bool) bool {
  	switch str, ok := attrs[key]; {
  	case !ok:
  		return defaultValue
  	case str == "true":
  		return true
  	case str == "false":
  		return false
  	default:
  		t.errorf("expected 'true' or 'false', got %q", str)
  	}
  	panic("")
  }
docs: // boolAttr returns a boolean value from the given attribute map.