theorem Th31:
  G1 == G2 & G1 is c-regular implies G2 is c-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, Lm2;
end;
