my( %one, %two );
    Tie::Hash::Complementary->marry( \%one, \%two );
    $one{foo} = 'bar';    # Also sets $two{bar}= 'foo'
    $two{baz} = 'bif';    # Also sets $one{bif}= 'baz'
    $two{bar} = 'oof';    # Does: $one{oof} = delete $one{foo}
    $one{oof} = 'baz';    # Dies; $two{baz} can't be both oof and bif