#lib() {
    .colors() {
        @primary: blue;
        @secondary: green;
    }
    .rules(@size) {
        border: @size solid white;
    }
}

.box when (#lib.colors[@primary] = blue) {
    width: 100px;
    height: ($width / 2);
}

.bar:extend(.box) {
    @media (min-width: 600px) {
        width: 200px;
        #lib.rules(1px);
    }
}