In this, MR Structs are introduced in a more general form than they are used in the index kernel. The structs here can hold data and pointers to fields. This makes it possible to iterate over a struct and extract field pointers in each loop iteration. The extracted fields are then updated in the normal loop nest.
Thus the struct is used as a container for an arbitrary number of subarrays that are all updated at once. Since the struct only holds a single pointer per Element in the above example we can represent it as a double pointer **
