theorem
  for D being set, F,G be XFinSequence of D^omega holds
    F c= G implies FlattenSeq F c= FlattenSeq G
proof
  let D be set, F,G be XFinSequence of D^omega;
  assume F c= G;
  then consider F9 being XFinSequence of D^omega such that
A1: F ^ F9 = G by Th79;
  FlattenSeq F ^ FlattenSeq F9 = FlattenSeq G by A1,Th74;
  hence thesis by AFINSQ_1:74;
end;
