
theorem
  for D being set, F,G be FinSequence of D* holds
  F c= G implies FlattenSeq F c= FlattenSeq G
proof
  let D be set, F,G be FinSequence of D*;
  assume F c= G;
  then consider F9 being FinSequence of D* such that
A1: F ^ F9 = G by FINSEQ_4:82;
  FlattenSeq F ^ FlattenSeq F9 = FlattenSeq G by A1,Th3;
  hence thesis by FINSEQ_6:10;
end;
