public int GetHashCode(TupleKey <TOuterKey, TInnerKey> obj) => obj.HashCode;
 public bool Equals(TupleKey <TOuterKey, TInnerKey> x, TupleKey <TOuterKey, TInnerKey> y)
 {
     return
         (_outerKeyComparer.Equals(x.OuterKey, y.OuterKey) &&
          _innerKeyComparer.Equals(x.InnerKey, y.InnerKey));
 }