<===> input.scss
.foo {
  yellow: yellow;
  red: red;
  blue: blue;
  white: white;
  black: black;
  eval: if(red, yellow, null);
}

<===> output.css
.foo {
  yellow: yellow;
  red: red;
  blue: blue;
  white: white;
  black: black;
  eval: yellow;
}