equals method
- E e1,
- E e2
override
    Compare two elements for being equal.
This should be a proper equality relation.
Implementation
@override
bool equals(E e1, E e2) {
  for (var eq in _equalities) {
    if (eq.isValidKey(e1)) return eq.isValidKey(e2) && eq.equals(e1, e2);
  }
  return false;
}