For completeness (and in case someone can spot my mistake!) here's the broken Y Combinator version:
(def Y2 (m) ((fn (f) (m (fn (a result) (f f) a result)))
(fn (f) (m (fn (a result) (f f) a result)))))
; broken
(def accumulate (combiner null-value term a next b)
((Y2 (fn (r)
(fn (a result)
(if (> a b) result
(r (next a) (combiner (term a) result))))))
a
null-value))
