reserve P,Q,X,Y,Z for set, p,x,x9,x1,x2,y,z for object;
reserve D for non empty set;

theorem Th93:
  for A being Subset of X holds (id X)"A = A
proof
  let A be Subset of X;
  thus A = (id X)"((id X).:A) by Th61
    .= (id X)"A by FUNCT_1:92;
end;
