lookup method

E lookup (Object element)
override

If an object equal to object is in the set, return it.

Checks if there is an object in the set that is equal to object. If so, that object is returned, otherwise returns null.

Implementation 

E lookup(Object element) {
  if (element == null) return null;

  return _sets
      .map((set) => set.lookup(element))
      .firstWhere((result) => result != null, orElse: () => null);
}