<% example do %>
  @mixin avatar($size, $circle: false) {
    width: $size;
    height: $size;

    @if $circle {
      border-radius: $size / 2;
    }
  }

  .square-av {
    @include avatar(100px, $circle: false);
  }
  .circle-av {
    @include avatar(100px, $circle: true);
  }
  ===
  @mixin avatar($size, $circle: false)
    width: $size
    height: $size

    @if $circle
      border-radius: $size / 2

  .square-av
    @include avatar(100px, $circle: false)
  .circle-av
    @include avatar(100px, $circle: true)
<% end %>