reserve a,a1,a2,v,v1,v2,x for object;
reserve V,A for set;
reserve m,n for Nat;
reserve S,S1,S2 for FinSequence;

theorem Th4:
  x in NDSS(V,A) implies x is TypeSSNominativeData of V,A
  proof
    assume x in NDSS(V,A);
    then ex w being TypeSSNominativeData of V,A st x = w;
    hence thesis;
  end;
