<===> options.yml
---
:todo:
- sass/libsass#2987

<===> input.scss
.m__exhibit-header--medium {
    @extend #{&}--plain;
    &--plain {
        font-size: --&;
    }
}
<===> output.css
.m__exhibit-header--medium--plain, .m__exhibit-header--medium {
  font-size: -- .m__exhibit-header--medium--plain;
}