@include test('clean(..)') {
  $tests: (
    '1.2.3': '1.2.3',
    ' 1.2.3 ': '1.2.3',
    ' 1.2.3-4 ': '1.2.3-4',
    ' 1.2.3-pre ': '1.2.3-pre',
    '  =v1.2.3   ': '1.2.3',
    'v1.2.3': '1.2.3',
    ' v1.2.3 ': '1.2.3',
    '=1.2.3': '1.2.3',
  );

  @each $test, $expect in $tests {
    @include assert-equal(clean($test), $expect, 'Gets rid off the leading `v`.');
  }
}