code: func interpolationFuncBase64Decode() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeString},
  		ReturnType: ast.TypeString,
  		Callback: func(args []interface{}) (interface{}, error) {
  			s := args[0].(string)
  			sDec, err := base64.StdEncoding.DecodeString(s)
  			if err != nil {
  				return "", fmt.Errorf("failed to decode base64 data '%s'", s)
  			}
  			return string(sDec), nil
  		},
  	}
  }
docs: // interpolationFuncBase64Decode implements the "base64decode" function that
  // allows Base64 decoding.