reserve i, j, k, l, m, n, t for Nat;

theorem Th8:
  i >= j implies j -' i = 0
proof
  assume
A1: i >= j;
  per cases by A1,XXREAL_0:1;
  suppose
    i = j;
    hence thesis by XREAL_1:232;
  end;
  suppose
    i > j;
    then j - i < 0 by XREAL_1:49;
    hence thesis by XREAL_0:def 2;
  end;
end;
