@function add($value1, $value2, $return-calc: true) {
  @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
  );
}

@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
  );
}