Yeah, I'm not seeing what's so special either. Maybe it's that you do have to specify that initial value, so your return types are never something you don't expect?
Huh. I did not know that. I am surprised that it actually coerces the result to an integer, although that's certainly an artificial type edge case.
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* (+ #c(10 -1) #c(20 1))
30
* (type-of (+ #c(10 -1) #c(20 1)))
(INTEGER 0 4611686018427387903)
For example, if you do this, you do get a complex number.