Buffer

`sig
type t
val create : int -> Buffer.t
val contents : Buffer.t -> string
val length : Buffer.t -> int
val clear : Buffer.t -> unit
val reset : Buffer.t -> unit
val add_char : Buffer.t -> char -> unit
val add_string : Buffer.t -> string -> unit
val add_substring : Buffer.t -> string -> int -> int -> unit
val add_substitute : Buffer.t -> (string -> string) -> string -> unit
val add_buffer : Buffer.t -> Buffer.t -> unit
val add_channel : Buffer.t -> Pervasives.in_channel -> int -> unit
val output_buffer : Pervasives.out_channel -> Buffer.t -> unit
end`