theorem
  for p being DTree-yielding FinSequence holds len doms p = len p
proof
  let p be DTree-yielding FinSequence;
A1: dom p = dom doms p by Th37;
  Seg len p = dom p by FINSEQ_1:def 3;
  hence thesis by A1,FINSEQ_1:def 3;
end;
