
theorem Th4:
  for n being Nat st n > 1 holds n -' 2 + 1 = n -' 1
proof
  let n be Nat;
  assume n > 1;
  then
A1: n -' 1 >= 1 by NAT_D:49;
  n -' 2 + 1 = n -' 1 -' 1 + 1 by NAT_D:45
    .= n -' 1 by A1,XREAL_1:235;
  hence thesis;
end;
