Accepts
an+b, odd, even, or a plain number.
Your selector
How the formula works
an+b steps through the list: a is the size of each step and b is the offset it starts from. n counts up from zero, so 3n+1 matches the 1st, 4th, 7th and so on. odd is shorthand for 2n+1, and even for 2n. A negative a counts backwards, which is how -n+5 selects only the first five.
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