let chain n = 
  let rec aux a = function
    | n when n <= 1 -> n::a
    | n when n mod 2 <> 0 -> aux (n :: a) (n * 3 + 1)
    | n -> aux (n :: a) (n / 2) in
  List.rev (aux [] n)
let p list = 
  let to_s list =
    "[" ^ String.concat ", " (List.map string_of_int list) ^ "]" in 
  print_newline (print_string (to_s list))
let range a b = 
  let rec aux a b acc =
    if b < a then acc else aux a (b - 1) (b :: acc) in  
  aux a b []
let f884 a b c = 
  List.length (List.filter (fun n -> c <= (List.length (chain n))) (range a b))
let () = p (chain 10)
let () = p (chain 30)
let () = print_int (f884 1 100 15)
 

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