This requires: ocaml -rectypes
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let st0 dmy = let st = "pusillanimous " in let l = (String.length st) - 1 in
   let rec s n = (st.[n], fun dum -> s (if n=l then 0 else n+1)) in s 0;;

let n = 4;;

type nodec = Lc of char | Nc of (nodec * nodec);;

let rec bt k st =
    if k=0 then match (st ()) with (ch, rs) -> (Lc ch, rs)
    else match bt (k-1) st with (trl, sty) ->
         match bt (k-1) sty with (trr, stz) -> (Nc (trl, trr), stz)
    in bt n st0;;