Append map slice.

main.go

package main

import (
	"fmt"
)

type foo struct {
	i int
}

func main() {
	m := map[int][]*foo{}
	m[1] = append(m[1], &foo{i: 123})
	m[1] = append(m[1], &foo{i: 456})
	fmt.Println(m)
	fmt.Println(m[1])
	fmt.Println(m[1][0])
	fmt.Println(m[1][1])
}