function iabs(const x: interval): interval;
  begin
    if (x.a <= 0) and (x.b <= 0) then
    begin
      Result.a := abs(x.b);
      Result.b := abs(x.a);
    end
    else if (x.a < 0) and (x.b > 0) then
    begin
      Result.a := 0;
      Result.b := max(abs(x.a), x.b);
    end
    else
    begin
      Result.a := x.a;
      Result.b := x.b;
    end;
  end { iabs };