code: func (m mountedFS) translate(path string) string {
  	path = pathpkg.Clean("/" + path)
  	if !hasPathPrefix(path, m.old) {
  		panic("translate " + path + " but old=" + m.old)
  	}
  	return pathpkg.Join(m.new, path[len(m.old):])
  }
docs: // translate translates path for use in m, replacing old with new.
  //
  // mountedFS{"/src/pkg", fs, "/src"}.translate("/src/pkg/code") == "/src/code".