reserve A for non empty set,
  a for Element of A;
reserve A for set;
reserve B,C for Element of Fin DISJOINT_PAIRS A,
  x for Element of [:Fin A, Fin A:],
  a,b,c,d,s,t,s9,t9,t1,t2,s1,s2 for Element of DISJOINT_PAIRS A,
  u,v,w for Element of NormForm A;
reserve K,L for Element of Normal_forms_on A;
reserve f,f9 for (Element of Funcs(DISJOINT_PAIRS A, [:Fin A,Fin A:])),
  g,h for Element of Funcs(DISJOINT_PAIRS A, [A]);

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;
