reserve A,B for Ordinal,
  K,M,N for Cardinal,
  x,x1,x2,y,y1,y2,z,u for object,X,Y,Z,X1,X2, Y1,Y2 for set,
  f,g for Function;

theorem
  M+`0 = M
proof
  thus M+`0 = card plus(M,{}) by Lm1
    .= card M by CARD_1:69
    .= M;
end;
