@function noop($string) {
  @return $string;
}

.foo {
  upper: to-upper-case('f') + str-slice('foo', 2);
  lower: to-lower-case('f') + str-slice('foo', 2);
  user-upper: to-upper-case('f') + noop('oo');
  user-lower: to-lower-case('f') + noop('oo');
}