+external head : t -> string -> string array = "ocaml_guestfs_head"
+external head_n : t -> int -> string -> string array = "ocaml_guestfs_head_n"
+external tail : t -> string -> string array = "ocaml_guestfs_tail"
+external tail_n : t -> int -> string -> string array = "ocaml_guestfs_tail_n"