reserve i,j,k,n for Nat;

theorem Th1:
  for i, n being Nat holds i <= n implies n - i + 1 is Element of NAT
proof
  let i, n be Nat;
  assume i <= n;
  then reconsider ni = n - i as Element of NAT by INT_1:5;
  ni + 1 is Element of NAT;
  hence thesis;
end;
