reserve f,g,h for Function,
  A for set;
reserve F for Function,
  B,x,y,y1,y2,z for set;

theorem
  (delta A)~ = delta A
proof
  thus (delta A)~ = <:id A, id A:>~ by Th1
    .= <:id A, id A:> by Th2
    .= delta A by Th1;
end;
