reserve n,m,k for Nat,
  x,y for set,
  r for Real;
reserve C,D for non empty finite set,
  a for FinSequence of bool D;

theorem Th4:
  for a be length_equal_card_of_set FinSequence of bool D holds len a <> 0
proof
  let A be length_equal_card_of_set FinSequence of bool D;
  assume len A = 0;
  then card(D)=0 by Th1;
  hence contradiction;
end;
