///Adds `$value` as first index of `$list`.
///@ignore Documentation: http://sassylists.com/documentation.html#sl-prepend
///@requires sl-is-true
///@requires sl-to-list
///@param {List} $list  - list to preprend value to
///@param {*}    $value - value to prepend to the list
///@example
///		sl-prepend(a b c, z)
///		// z a b c
///
///@return {List}
 
@function sl-prepend($list, $value) {
  @if sl-missing-dependencies(sl-is-true, sl-to-list) == true { @return null; }
  
  @if sl-is-true($value) {
  	$list: join($value, $list, list-separator($list));
  }

  @return sl-to-list($list);
}