
theorem Th20:
  for T be full Tree for n be non zero Nat holds dom
  FinSeqLevel(n,T) = Seg (2 to_power n)
proof
  let T be full Tree;
  let n be non zero Nat;
  thus dom FinSeqLevel(n,T) = Seg len FinSeqLevel(n,T) by FINSEQ_1:def 3
    .= Seg (2 to_power n) by Th19;
end;
