title: Put http.NewServeMux into http.NewServeMux
body: <p>This is my code. I want to put mux1 as sub router into mux.Handle. Next, I run the code. I can visit path /index but I cannot visit path /index/sub1.
  I don't know why I can visit /index but I cannot visit /index/sub1?</p>
  
  <pre><code>package main
  
  import (
      "io"
      "net/http"
  )
  
  func main() {
      mux1 := http.NewServeMux()
      mux1.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
          io.WriteString(w, "sub index")
      })
      mux1.HandleFunc("/sub1", func(w http.ResponseWriter, r *http.Request) {
          io.WriteString(w, "sub 1")
      })
  
      mux := http.NewServeMux()
      mux.Handle("/index", mux1)
  
      http.ListenAndServe(":8000", mux)
  }
  </code></pre>