code: public static <T> SortedSet<T> plus(SortedSet<T> left, T right) {
          return (SortedSet<T>) plus((Collection<T>) left, right);
      }
docs: Create a SortedSet as a union of a SortedSet and an Object.
  This operation will always create a new object for the result,
  while the operands remain unchanged.
  <pre class="groovyTestCase">assert [1,2,3] == [1,2] + 3</pre>
  
  @param left  a SortedSet
  @param right an object to add/append
  @return the resulting SortedSet
  @since 2.4.0