Description: The union of two pseudographs (with the same vertex set): If
<. V , E >. and <. V , F >. are pseudographs, then
<. V , E u. F >. is a pseudograph (the vertex set stays the same,
but the edges from both graphs are kept). (Contributed by Mario
Carneiro, 12-Mar-2015)(Revised by AV, 12-Oct-2020)(Revised by AV, 24-Oct-2021)