zipper_b.fs

let rec iterate fn prop start =
    if prop start then start
    else iterate fn prop (fn start)

let rec next node =
    if is_left node then node |> right |> up |> firstChild
    else node |> iterate up is_left |> next

let rec delete n off node =
    let newnode = node |> toString |> boundedsubstring n off |> fromString
    let rem = n + off - length node
    if rem > 0 then delete rem 0 (next newnode)
    else newnode

let rec insert off node text =
    let left = (subrope 0 off node)
    let right = (subrope off (len node - off) node)
    concat left (concat text right)