Returns both the popped element and the remaining sequence.
Examples
s <- as_flexseq(letters[1:4])
out <- pop_at(s, 3)
out$element
#> [1] "c"
out$remaining
#> FingerTree <size=3, named=no>
#> monoids: none
#>
#> [[1]]
#> [1] "a"
#>
#> [[2]]
#> [1] "b"
#>
#> [[3]]
#> [1] "d"
#>