How to add new methods to an existing type in Go?

I want to add a convenience util method on to gorilla/mux Route and Router types:

package util

import(
    "net/http"
    "github.com/0xor1/gorillaseed/src/server/lib/mux"
)

func (r *mux.Route) Subroute(tpl string, h http.Handler) *mux.Route{
    return r.PathPrefix("/" + tpl).Subrouter().PathPrefix("/").Handler(h)
}

func (r *mux.Router) Subroute(tpl string, h http.Handler) *mux.Route{
    return r.PathPrefix("/" + tpl).Subrouter().PathPrefix("/").Handler(h)
}

but the compiler informs me

Cannot define new methods on non-local type mux.Router

So how would I achieve this? Do I create a new struct type that has an anonymous mux.Route and mux.Router fields? Or something else?

As the compiler mentions, you can't extend existing types in another package. You can define your own type backed by the original as follows:

type MyRouter mux.Router

func (m *MyRouter) F() { ... }

or by embedding the original router:

type MyRouter struct {
    *mux.Router
}

func (m *MyRouter) F() { ... }

...
r := &MyRouter{router}
r.F()