Quantity queries mixins

_quantity-queries.scss

@mixin at-least($count) {
    @if type-of($count) != 'number' {
        @error '`#{$count}` is not a valid number for at-least() mixin.';
    }
    @at-root &:nth-last-child(n+#{$count}),
    &:nth-last-child(n+#{$count}) ~ * {
        @content;
    }
}

@mixin at-most($count) {
    @if type-of($count) != 'number' {
        @error '`#{$count}` is not a valid number for at-most() mixin.';
    }
    @at-root &:nth-last-child(-n+#{$count}):first-child,
    &:nth-last-child(-n+#{$count}):first-child ~ * {
        @content;
    }
}

@mixin between($first, $last) {
    @if $first > $last {
        @error '#{$first} can´t be larger that #{$last} for in-between() mixin';
    }

    @include at-least($first) {
        @include at-most($last) {
            @content;
        }
    }
}

test.scss

.nav li {

    // If nav containes 3 items or less, make them green.
    @include at-most(3) {
        color: green;
    }

    // If nav containes 6 items or more, make-them red red.
    @include at-least(6) {
        color: red;  
    }

            // If nav contains between 4 and 6 items, add a blue background.
    @include between(4, 6) {

        background-color: rgba(blue, .1);
    }
}