Hence my "but hmm." ^_^

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"I'll note that (del (foo 0)) is equivalent to (= (foo 0 1) nil) so perhaps that confused you."

Yeah, that's probably it.

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"I always thought of it as being a start/end position, like cut."

It's hard to tell because you use 0 as the start pretty often, but here are the two examples I was going off of:

From http://arclanguage.org/item?id=14656:

  (= foo '(1 2 3 4 5 6 7))

  (del (foo 1 4)) -> (1 6 7)

  (= foo '(1 2 3 4))

  (del (foo 1 2)) -> (1 4)