<===> input.scss
@if true {
  div {
    background: green;
  }
}
@if not true {
  div {
    background: red;
  }
}
@if not not true {
  div {
    background: blue;
  }
}
@if not (true or false) {
  div {
    background: black;
  }
}
<===> output.css
div {
  background: green;
}

div {
  background: blue;
}