<===> input.scss
@if 2 {
  div {
    background: green;
  }
}
@if not 2 {
  div {
    background: red;
  }
}
@if not not 2 {
  div {
    background: blue;
  }
}

<===> output.css
div {
  background: green;
}

div {
  background: blue;
}