Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="box-grey"></div>
<div class="box-rounded"></div>

SassMeister-input.scss

// ----
// Sass (v3.4.13)
// Compass (v1.0.3)
// ----

@mixin module($name, $args: ()) {
  $i: ".";
  @if map-get($args, "silent") { $i: "%"; }

  #{$i}#{$name} {
    @if map-get($args, "extend") {
      @extend .#{map-get($args, "extend")} !optional;
      @extend %#{map-get($args, "extend")} !optional;
    }
    @content;
  }
}

// you can also make the abstract one to be silent
// @include module("abstract-box", (silent:true))
@include module("abstract-box") {
  width: 50px;
  height: 50px;
  background: black;
  border: 1px solid black;
  float: left;
  margin-right: 10px;
}

@include module("box-grey", (extend:"abstract-box")) {
  background: #CCC;
}
@include module("box-rounded", (extend:"abstract-box")) {
  border-radius: 10px;
}

SassMeister-output.css

.abstract-box, .box-grey, .box-rounded {
  width: 50px;
  height: 50px;
  background: black;
  border: 1px solid black;
  float: left;
  margin-right: 10px;
}

.box-grey {
  background: #CCC;
}

.box-rounded {
  border-radius: 10px;
}

SassMeister-rendered.html

<div class="box-grey"></div>
<div class="box-rounded"></div>