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

theorem Th20:
  G is c1-regular c2-regular implies c1 = c2
proof
  assume A1: G is c1-regular c2-regular;
  set v = the Vertex of G;
  thus c1 = v.degree() by A1
    .= c2 by A1;
end;
