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
  Atom(A).a = singleton DISJOINT_PAIRS A .a
proof
  thus (singleton DISJOINT_PAIRS A).a = {a} by SETWISEO:54
    .= Atom A.a by Def4;
end;
