For example, suppose you want to write a selector that matches the outer
selector *and* an element selector. You could write a mixin like this one that
uses the [`selector-unify()` function][] to combine `&` with a user's selector.

[`selector-unify()` function]: ../functions/selector#selector-unify

<% example do %>
  @mixin unify-parent($child) {
    @at-root #{selector-unify(&, $child)} {
      @content;
    }
  }

  .wrapper .field {
    @include unify-parent("input") {
      /* ... */
    }
    @include unify-parent("select") {
      /* ... */
    }
  }
  ===
  @mixin unify-parent($child)
    @at-root #{selector-unify(&, $child)}
      @content



  .wrapper .field
    @include unify-parent("input")
      /* ...

    @include unify-parent("select")
      /* ...
<% end %>