code: func Map(m map[string][]byte) FileSystem {
  	for k := range m {
  		if strings.HasPrefix(k, "/") {
  			panic("filename has illegal leading slash: " + k)
  		}
  	}
  
  	return mapFS(m)
  }
docs: // Map returns a new FileSystem from the provided map. Map keys
  // should be forward slash-separated pathnames and not contain a
  // leading slash.
  //
  // It differs from golang.org/x/tools/godoc/vfs/mapfs in the following
  // ways:
  //
  // * It uses []byte instead of string to hold file contents. This
  //   means that callers must synchronize writes (if any) to the byte
  //   arrays.
  // * It implements ctxvfs.FileSystem, not vfs.FileSystem.