Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.3.4)
// Compass (v1.0.0.alpha.18)
// ----

@mixin featureFlag ($flag) {
  @at-root {
   .#{$flag} & {
    @content;
   }
 } 
}

.parent {
  width: 10px;
  @include featureFlag(ie7) {
    height: 3px;
    @include featureFlag(hoverable) {
      .thing {
        color: red;
      }
    }
  }
}

SassMeister-output.css

.parent {
  width: 10px;
}
.ie7 .parent {
  height: 3px;
}
.hoverable .ie7 .parent .thing {
  color: red;
}