Iterate over multiple enumerators in parallel, using the external interface based on the #next method. Each call to #next returns an array, containing the next element for each of the enumerators. A StopIteration exception is raised as soon as any of the enumerators runs out of elements. SyncEnum differs from the standard library's REXML::SyncEnumerator in its use of the #next external iterator interface, while REXML::SyncEnumerator uses an #each internal interator interface. The external interface is more convenient when you expect to end iteration before reaching the end of any of the enumerations, including cases where an enumerator generates an unending sequence.