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