Suppose we created a multidimensional array, with as many dimensions as
we have sets, and in which the size of each dimension were equal to the size of
the corresponding set. That array would have exactly as many elements as the
cross product of all these sets, and we could easily put the elements of the
cross product into the array by stepping the index for each dimension through
each of the sets. But this array notion also gives a unique index to every
element of the cross product. This function works as if we created such an
array and then did an `array-row-major-aref' into it.