Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.1)
// Compass (v1.0.0.alpha.18)
// ----

// components
%box-list {
  display: block;

  &__item {
    display: block;
    margin:  0;
    padding: 0;
    
    &__link {
      display: block;
      color: blue;
    }
  }
}
.box-list {
  @extend %box-list;
  .item {
    @extend %box-list__item;
    & > a { @extend %box-list__item__link; }
  }
}

// module
%nav-list {
  @extend %box-list;
  
  &__item {
    @extend %box-list__item;
    
    & > a {
      @extend %box-list__item__link;
    }
  }
  
  &--horizontal {
    display: box;
    box-orient: horizontal;
    box-flex: 1;    
  }
}

.nav-list { // ul or olを想定
  @extend %nav-list;
  
  & > li {
    @extend %nav-list__item;
  }
}

// theme
.site-nav {
  @extend %nav-list;
  @extend %nav-list--horizontal;
  
  & > li {
    @extend %nav-list__item;
  }
}

SassMeister-output.css

.box-list, .nav-list, .site-nav {
  display: block;
}
.box-list .item, .nav-list > li, .site-nav > li {
  display: block;
  margin: 0;
  padding: 0;
}
.box-list .item > a, .nav-list > li > a, .site-nav > li > a {
  display: block;
  color: blue;
}

.site-nav {
  display: box;
  box-orient: horizontal;
  box-flex: 1;
}