code: public static <T> Set<T> grep(Set<T> self) {
          return grep(self, Closure.IDENTITY);
      }
docs: Iterates over the collection returning each element that matches
  using the IDENTITY Closure as a filter - effectively returning all elements which satisfy Groovy truth.
  <p>
  Example:
  <pre class="groovyTestCase">
  def items = [1, 2, 0, false, true, '', 'foo', [], [4, 5], null] as Set
  assert items.grep() == [1, 2, true, 'foo', [4, 5]] as Set
  </pre>
  
  @param self a Set
  @return a Set of elements satisfy Groovy truth
  @see Closure#IDENTITY
  @since 2.4.0