code: func interpolationFuncLower() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{ast.TypeString},
  		ReturnType: ast.TypeString,
  		Callback: func(args []interface{}) (interface{}, error) {
  			toLower := args[0].(string)
  			return strings.ToLower(toLower), nil
  		},
  	}
  }
docs: // interpolationFuncLower implements the "lower" function that does
  // string lower casing.