## ยง CP trick: writing exact counting as counting less than

- If we can solve for number of elements
`<= k`

, say given by `leq(k)`

where `k`

is an integer, then we can also solve for number of elements `= k`

, given by `eq(k) := leq(k) - leq(k - 1)`

. - While simple, this is hugely benificial in many situations because
`<=k`

can be implement as some kind of prefix sum data structure plus binary search, which is much less error prone to hack up than exact equality.