Description: An edge of a simple graph always connects two vertices. Analogue of usgredgprv . (Contributed by Alexander van der Vekens, 7-Oct-2017) (Revised by AV, 9-Jan-2020) (Revised by AV, 23-Oct-2020) (Proof shortened by AV, 27-Nov-2020)
|- E = ( Edg ` G )
|- V = ( Vtx ` G )
|- ( ( G e. USGraph /\ { M , N } e. E ) -> ( M e. V /\ N e. V ) )
|- ( G e. USGraph -> G e. UMGraph )
|- ( ( G e. UMGraph /\ { M , N } e. E ) -> ( M e. V /\ N e. V ) )