<===> input.scss
// Make sure compound selectors are unified when two :not()s are extended.
// :not() is special here because it's the only selector that's extended by
// adding to the compound selector, rather than creating a new selector list.
.a {@extend .c}
.b {@extend .d}
:not(.c):not(.d) {x: y}

<===> output.css
:not(.c):not(.a):not(.d):not(.b) {
  x: y;
}