theorem Th7:
  for A, B being set st A c= B holds NDSS(V,A) c= NDSS(V,B)
  proof
    let A,B be set such that
A1: A c= B;
    let d be object;
    assume d in NDSS(V,A);
    then ex w being TypeSSNominativeData of V,A st w = d;
    then d is TypeSSNominativeData of V,B by A1,RELSET_1:7;
    hence thesis;
  end;
