equals method

bool equals (E e1, E e2)
override

Compare two elements for being equal.

This should be a proper equality relation.

Implementation 

bool equals(E e1, E e2) {
  for (Equality<E> eq in _equalities) {
    if (eq.isValidKey(e1)) return eq.isValidKey(e2) && eq.equals(e1, e2);
  }
  return false;
}