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;

theorem Th5:
  for X being set st X c= u holds X is Element of NormForm A
proof
  let X be set;
  assume
A1: X c= u;
  u = @u;
  then X in Normal_forms_on A by A1,Th4;
  hence thesis by NORMFORM:def 12;
end;
