theorem
 for X being FinSequence-membered set ex Y being non empty set st X c= Y*
proof
 let X be FinSequence-membered set;
  set Z = {rng f where f is Element of X: f in X};
 take Y = succ union Z;
 let x be object;
 assume
A1: x in X;
  then reconsider x as FinSequence by Def18;
  rng x in {rng f where f is Element of X: f in X} by A1;
  then rng x c= Y by ORDINAL3:1,SETFAM_1:41;
  then x is FinSequence of Y by Def4;
 hence thesis by Def11;
end;
