theorem Th13:
  (maxfuncreal(A)).((minfuncreal(A)).(f,g),f)=f
proof
  thus (maxfuncreal(A)).((minfuncreal(A)).(f,g),f) =(maxfuncreal(A)).(f,(
  minfuncreal(A)).(f,g)) by Th8
    .=f by Th12;
end;
