theorem Th35:
  l <> 0 implies l divides l!
proof
  assume l<>0;
  then consider n being Nat such that
A1: l = n+1 by NAT_1:6;
  reconsider n as Element of NAT by ORDINAL1:def 12;
  (n+1)! = (n+1) * (n!) by Th15;
  hence thesis by A1,NAT_D:def 3;
end;
