@include test-module('_add') {
    @include test('should add two numbers together') {
        @include assert-equal(_add(6, 4), 10);
    }

    @include test('should coerce numerical strings to numbers') {
        @include assert-equal(_add(6, '14'), 20);
    }

    @include test('should be chainable') {
        @include assert-equal(_(1, _add 2), 3);
    }

    @include test('should work with units') {
        @include assert-equal(_add(6px, 4px), 10px);

        @include assert-equal(_add(10%, 20%), 30%);
    }

    @include test('should work across similar units') {

        @include assert-equal(_add(3s, 101ms), 3.101s);

        @include assert-equal(_add(101ms, 3s), 3101ms);

        @include assert-equal(_add(6px, 4), 10px);
    }

    @include test('should work using calc() across different units') {
        @include assert-equal(_add(3px, 5rem), calc(3px + 5rem));
    }

    @include test('should use calc() across units of unknown unit groups') {
        @include assert-equal(_add(3rem, 5ch), calc(3rem + 5ch));

        @include assert-equal(_add(3px, 5ex), calc(3px + 5ex));

        @include assert-equal(_add(3dppi, 5px), calc(3dppi + 5px));
    }

    @include test('should not use calc() if any unit is an empty unit') {
        @include assert-equal(_add(3px, 5), 8px);

        @include assert-equal(_add(3, 5), 8);
    }

    @include test('should work with calc() and negative values') {
        @include assert-equal(_add(3rem, -80%), calc(3rem + -80%));
    }

    @include test('should be aliased') {
        @include assert-equal(\+(3px, 6px), _add(3px, 6px));
    }
}