code: public static Object sum(Iterator<Object> self, Closure closure) {
          return sum(toList(self), null, closure, true);
      }
docs: Sums the result of apply a closure to each item returned from an iterator.
  <code>iter.sum(closure)</code> is equivalent to:
  <code>iter.collect(closure).sum()</code>. The iterator will become
  exhausted of elements after determining the sum value.
  
  @param self    An Iterator
  @param closure a single parameter closure that returns a numeric value.
  @return The sum of the values returned by applying the closure to each
  item from the Iterator.
  @since 1.7.1