"In the normal case, the result of reduce is the combined result of function's being applied to successive pairs of elements of sequence. If the subsequence contains exactly one element and no initial-value is given, then that element is returned and function is not called. If the subsequence is empty and an initial-value is given, then the initial-value is returned and function is not called. If the subsequence is empty and no initial-value is given, then the function is called with zero arguments, and reduce returns whatever function does. This is the only case where the function is called with other than two arguments."

True, but sometimes you want some particular value for those special cases. In case of a sum (reduce add ()) should return 0, which is the (mathematically) "right" result.

Yes, I suppose the real solution is an optional initial value argument (as in CL reduce)), thus you can provide an appropriate identity value (e.g. 0 in the case of add).