code: public static <T> Set<T> leftShift(Set<T> self, T value) {
          return (Set<T>) leftShift((Collection<T>) self, value);
      }
docs: Overloads the left shift operator to provide an easy way to append
  objects to a Set.
  <pre class="groovyTestCase">def set = [1,2] as Set
  set &lt;&lt; 3
  assert set == [1,2,3] as Set</pre>
  
  @param self  a Set
  @param value an Object to be added to the Set.
  @return same Set, after the value was added to it.
  @since 2.4.0