Background:

The call

(array-inner-product A f g B)

produces a (generalized) array; elements of that array are computed by
reducing g on the result of mapping f along the last axis of A and the
first axis of B.

Reducing on an empty array (i.e., when the last axis of A and the first
axis of B are empty) is an error.  The code for array-inner-product
always recognized that, but didn't throw an error until an element of
(array-inner-product A f g B) is accessed.  If you didn't access any
elements of the result, the error wasn't thrown.

After discussions on the Chibi Github issues page, I changed the code to
throw an error earlier, when checking the arguments to the call to
array-inner-product.

The issue:

The document doesn't say that it is an error if the last axis of
(array-domain A) is empty.

I'd like to correct this omission.

Brad