let G1, G2 be _Graph; :: thesis: ( G1 == G2 & G1 is regular implies G2 is regular )
assume G1 == G2 ; :: thesis: ( not G1 is regular or G2 is regular )
then consider F being PGraphMapping of G1,G2 such that
A1: ( F = id G1 & F is Disomorphism ) by GLIBPRE0:75;
thus ( not G1 is regular or G2 is regular ) by A1, Th40; :: thesis: verum