reserve x,y,z for object, X,Y for set,
  i,k,n for Nat,
  p,q,r,s for FinSequence,
  w for FinSequence of NAT,
  f for Function;

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;
