Okay, I've made up my mind. The "err" option is not tenable because it can cloak real exceptions, and having multiple versions of reduce is simply multiplying entities without adding much power. So let's allow an optional "identvalue" trait on operators. If it's there, reduce can use it. If it's not, reduce returns failure on 0 args. Built-in addition will have an identity value of 0, while multiplication will have an identity value of 1. String concatenation will have "". We can go as far as having -Inf on [<] and +Inf on [>], but maybe that means we have to define values that are positive and negative infinity on strings so we can get [gt] and [lt] indentity values. Those might be useful in any event.
But I would like to stress that this is an *optional* property. We will only define an identity value for those operators for which it is obvious what an "accumulator" should be initialized to. (This is probably also the set of operators for which op= doesn't warn when the left side is undefined, so "identval" is doing double duty. In Perl 5 we hardwired that, so it'd be nice to generalize the concept in Perl 6.) In fact, by the accumulator argument, maybe it should be called "initvalue" rather than "identvalue". Larry