@function subtract($value1, $value2, $return-calc: true) {
  @if $value1 == null and $value2 == null {
    @return null;
  }

  @if $value1 == null {
    @return -$value2;
  }

  @if $value2 == null {
    @return $value1;
  }

  @if type-of($value1) == number and type-of($value2) == number and comparable($value1, $value2) {
    @return $value1 - $value2;
  }

  @return if($return-calc == true, calc(#{$value1} - #{$value2}), $value1 + unquote(" - ") + $value2);
}