theorem
  p is FinSequence of D implies p - A is FinSequence of D
proof
  assume p is FinSequence of D;
  then
A1: rng p c= D by FINSEQ_1:def 4;
  rng(p - A) = rng p \ A by Th63;
  then rng(p - A) c= D by A1;
  hence thesis by FINSEQ_1:def 4;
end;
