reserve c,c1,c2 for Cardinal, G,G1,G2 for _Graph, v for Vertex of G;

theorem
  G1 == G2 & G1 is regular implies G2 is regular
proof
  assume G1 == G2;
  then consider F being PGraphMapping of G1, G2 such that
    A1: F = id G1 & F is Disomorphism by GLIBPRE0:75;
  thus thesis by A1, Th40;
end;
