code: public void link(Name name, File file) {
      DirectoryEntry entry = new DirectoryEntry(this, checkNotReserved(name, "link"), file);
      put(entry);
      file.linked(entry);
    }
docs: Links the given name to the given file in this directory.
  
  @throws IllegalArgumentException if {@code name} is a reserved name such as "." or if an entry
  already exists for the name