code: public static Object sum(Object[] self, Object initialValue, Closure closure) {
          return sum(toList(self), initialValue, closure, false);
      }
docs: Sums the result of applying a closure to each item of an array to some initial value.
  <code>array.sum(initVal, closure)</code> is equivalent to:
  <code>array.collect(closure).sum(initVal)</code>.
  
  @param self         an array
  @param closure      a single parameter closure that returns a numeric value.
  @param initialValue the closure results will be summed to this initial value
  @return The sum of the values returned by applying the closure to each
  item of the array.
  @since 1.7.1