code: public static Object sum(Iterable self, Object initialValue) {
          return sum(self, initialValue, false);
      }
docs: Sums the items in an Iterable, adding the result to some initial value.
  <pre class="groovyTestCase">
  assert 5+1+2+3+4 == [1,2,3,4].sum(5)
  </pre>
  
  @param self         an Iterable of values to sum
  @param initialValue the items in the collection will be summed to this initial value
  @return The sum of all of the items.
  @since 2.2.0