theorem Th25:
  [\ r /] = r iff r is integer
proof
  r is Integer implies [\ r /] = r
  proof
    r + 0 < r + 1 by XREAL_1:6;
    then r - 1 < r + 1 - 1 by XREAL_1:9;
    hence thesis by Def6;
  end;
  hence thesis;
end;
