theorem Th3: for D1, D2 being set st D1 c= D2 holds D1 ** c= D2 **
  proof
    let D1, D2 be set;
    assume
A1: D1 c= D2;
    let x be object;
    assume x in D1 **;
    then reconsider p = x as one-to-one FinSequence of D1 by Def2;
    rng p c= D2 by A1;
    then x is one-to-one FinSequence of D2 by FINSEQ_1:def 4;
    hence x in D2 ** by Def2;
  end;
