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