@mixin one-positional-arg($a,) {
  one-positional-arg: positional 1 $a;
}

@mixin two-positional-args($a, $b,) {
  two-positional-args: positional 2 $a $b;
}

@mixin one-keyword-arg($a: a,) {
  one-keyword-arg: keyword 1 $a;
}

@mixin two-keyword-args($a: a, $b: b,) {
  two-keyword-args: keyword 2 $a $b;
}

@mixin mixed-args($a, $b: b,) {
  mixed-args: keyword 2 $a $b;
}

.includes {
  @include one-positional-arg(a,);
  @include two-positional-args(a,b,);
  @include one-keyword-arg($a: z,);
  @include two-keyword-args($a: y,$b: z,);
  @include mixed-args(y, $b: z,);
}