### code.ml [ocaml]
class normalization_cmd () =
  let no_normalization _ = 1.
  and tree_length t = Gtree.tree_length t
  in
  let normalization_map =
    StringMap.of_pairlist
      [
        "", no_normalization;
        "tree-length", tree_length;
      ]
  in
  object
    val normalize = flag "--normalize"
      (Plain ("", "Divide KR by a given value. Legal arguments are \"tree-length\"."))
    method specl = [ string_flag normalize; ]

  method get_normalization: 'a. (#Newick_bark.newick_bark Gtree.gtree as 'a) -> float =
    let s = fv normalize in
    try StringMap.find s normalization_map
    with
      | Not_found -> failwith ("Normalization "^s^" not known.")
end
### error.txt
File "pplacer_src/guppy_cmdobjs.ml", line 203, characters 28-85:
Error: This type scheme cannot quantify 'a :
it is not a variable.