Horizontal stretchy operator, embellished with two nested mrows (reference)

This test passes if you see green rectangles and no red.

\underset{}{⥚}

\underset{⥚}{}

\overset{}{⥚}

\overset{⥚}{}

⥚_{}^{}

_{⥚}^{}

_{}^{⥚}

⥚_{}^{⥚}

⥚_{}^{⥚}

_{⥚}^{⥚}

⥚_{⥚}^{⥚}