How can I add custom functon?

[Caratpine]

Hi, masters
I found this #178 talking about this feature
May I ask if this feature can be supported currently?
I try to write a demo

package main

import (
	"fmt"
	"github.com/hyperjumptech/grule-rule-engine/ast"
	"github.com/hyperjumptech/grule-rule-engine/builder"
	"github.com/hyperjumptech/grule-rule-engine/engine"
	"github.com/hyperjumptech/grule-rule-engine/pkg"
	"time"
)

type MyFact struct {
	IntAttribute     int64
	StringAttribute  string
	BooleanAttribute bool
	FloatAttribute   float64
	TimeAttribute    time.Time
	WhatToSay        string
	UserSourceIP     []string
}

func println(s string) {
	fmt.Println("test", s)
}

func main() {
	var err error
	fact := &MyFact{
		IntAttribute:     123,
		StringAttribute:  "Some string value",
		BooleanAttribute: true,
		FloatAttribute:   1.234,
		TimeAttribute:    time.Now(),
	}

	dataCtx := ast.NewDataContext()
	if err = dataCtx.Add("MF", fact); err != nil {
		panic(err)
	}
	if err = dataCtx.Add("println", println); err != nil {
		panic(err)
	}
	drls := `
	rule CheckValues "Check the default values" salience 10 {
		when 
			MF.IntAttribute == 123 && MF.StringAttribute == "Some string value"
		then
			println(MF.StringAttribute);
			Retract("CheckValues");
	}`
	knowledgeLibrary := ast.NewKnowledgeLibrary()
	ruleBuilder := builder.NewRuleBuilder(knowledgeLibrary)
	bs := pkg.NewBytesResource([]byte(drls))
	err = ruleBuilder.BuildRuleFromResources("test", "0.0.1", []pkg.Resource{bs})
	if err != nil {
		fmt.Println(err.Error())
		panic(err)
	}
	knowledgeBase, _ := knowledgeLibrary.NewKnowledgeBaseInstance("test", "0.0.1")
	eng := engine.NewGruleEngine()
	err = eng.Execute(dataCtx, knowledgeBase)
	fmt.Println(fact.WhatToSay)
}

but it raised an error like this:

ERRO[0000] error while executing expression println(MF.StringAttribute). got this node identified as "DEFUNC" have no function named println  lib=grule-rule-engine package=ast
ERRO[0000] Failed execution rule : CheckValues. Got error this node identified as "DEFUNC" have no function named println  lib=grule-rule-engine package=engine