theorem Th17:
  {[{},{}]} is Element of Normal_forms_on A
proof
  [{},{}] is Element of DISJOINT_PAIRS A by Th12;
  then {[{},{}]} c= DISJOINT_PAIRS A by ZFMISC_1:31;
  then reconsider B = {[{},{}]} as Element of Fin DISJOINT_PAIRS A by
FINSUB_1:def 5;
  now
    let a,b be Element of DISJOINT_PAIRS A;
    assume that
A1: a in B and
A2: b in B and
    a c= b;
    a = [{},{}] by A1,TARSKI:def 1;
    hence a = b by A2,TARSKI:def 1;
  end;
  hence thesis by NORMFORM:33;
end;
