@function __once($function) {
    @return __before($function, 2);
}


/// Creates a function that is restricted to invoking `$function` once. Repeat calls
/// to the function return the value of the first call. The `$function` is invoked
/// with the `_this` binding of the created function.
///
///
/// @access public
/// @group Function
/// @param {Function} $func The function to restrict.
/// @returns {Function} Returns the new restricted function.
/// @example scss
/// $lighten-once: _once(lighten);
/// $color: _exec($lighten-once, green, 10%);
/// 
/// color: $color; // lightened green
/// 
/// $color: _exec($lighten-once, blue, 20%);
/// 
/// color: $color; // null

@function _once($args...) {
    @return call(get-function('__once'), $args...);
}