His formulation of reduce() is strikingly clumsy, both in signature and implementation. I daresay I wouldn't have much use for such a function either!
Most languages which provide a reduce() permit programmers to provide an initial "carry-in" value. This is a neater and more useful way to handle the cases of a zero- or one-element list. Moreover, it lets you do more interesting things with the reduction. Consider the following, using ES6-style JavaScript to collect a set of the unique values of a list via reduce():
function unique(list) {
return list.reduce(function (sofar, item) {
if (!sofar.includes(item)) { sofar.push(item); }
return sofar;
}, []);
}
