I'd like to point out that there is no default value for array-reduce on
empty arrays for any operator.  I couldn't figure out a reasonable way
to provide one, short of something like an alist with

+ -> 0
fx+ -> 0
fl+ -> 0.0
* -> 1
fx* -> 1
fl* -> 1.0

etc.  (Although I can't imagine right now what other entries might be in
the table.) This affects array-inner-product, too.

array-fold[l|r] returns what I call the "identity" argument for empty
arrays.  But you can't pass such an argument to reduce.

Brad