module String:String operations.sig..end
  Given a string s of length l, we call character number in s
  the index of a character in s.  Indexes start at 0, and we will
  call a character number valid in s if it falls within the range
  [0...l-1]. A position is the point between two characters or at
  the beginning or end of the string.  We call a position valid
  in s if it falls within the range [0...l]. Note that character
  number n is between positions n and n+1.
  Two parameters start and len are said to designate a valid
  substring of s if len >= 0 and start and start+len are
  valid positions in s.
  OCaml strings can be modified in place, for instance via the
  String.set and String.blit functions described below.  This
  possibility should be used rarely and with much care, however, since
  both the OCaml compiler and most OCaml libraries share strings as if
  they were immutable, rather than copying them.  In particular,
  string literals are shared: a single copy of the string is created
  at program loading time and returned by all evaluations of the
  string literal.  Consider for example:
        # let f () = "foo";;
      val f : unit -> string = <fun>
      # (f ()).[0] <- 'b';;
      - : unit = ()
      # f ();;
      - : string = "boo"
  
  Likewise, many functions from the standard library can return string
  literals or one of their string arguments.  Therefore, the returned strings
  must not be modified directly.  If mutation is absolutely necessary,
  it should be performed on a fresh copy of the string, as produced by
  String.copy.
val length : string -> intval get : string -> int -> charString.get s n returns character number n in string s.
   You can also write s.[n] instead of String.get s n.
   Raise Invalid_argument if n not a valid character number in s.
val set : string -> int -> char -> unitString.set s n c modifies string s in place,
   replacing the character number n by c.
   You can also write s.[n] <- c instead of String.set s n c.
   Raise Invalid_argument if n is not a valid character number in s.
val create : int -> stringString.create n returns a fresh string of length n.
   The string initially contains arbitrary characters.
   Raise Invalid_argument if n < 0 or n > Sys.max_string_length.
val make : int -> char -> stringString.make n c returns a fresh string of length n,
   filled with the character c.
   Raise Invalid_argument if n < 0 or n > Sys.max_string_length.
val copy : string -> stringval sub : string -> int -> int -> stringString.sub s start len returns a fresh string of length len,
   containing the substring of s that starts at position start and
   has length len.
   Raise Invalid_argument if start and len do not
   designate a valid substring of s.
val fill : string -> int -> int -> char -> unitString.fill s start len c modifies string s in place,
   replacing len characters by c, starting at start.
   Raise Invalid_argument if start and len do not
   designate a valid substring of s.
val blit : string -> int -> string -> int -> int -> unitString.blit src srcoff dst dstoff len copies len characters
   from string src, starting at character number srcoff, to
   string dst, starting at character number dstoff. It works
   correctly even if src and dst are the same string,
   and the source and destination intervals overlap.
   Raise Invalid_argument if srcoff and len do not
   designate a valid substring of src, or if dstoff and len
   do not designate a valid substring of dst.
val concat : string -> string list -> stringString.concat sep sl concatenates the list of strings sl,
   inserting the separator string sep between each.val iter : (char -> unit) -> string -> unitString.iter f s applies function f in turn to all
   the characters of s.  It is equivalent to
   f s.[0]; f s.[1]; ...; f s.[String.length s - 1]; ().val iteri : (int -> char -> unit) -> string -> unitString.iter, but the
   function is applied to the index of the element as first argument
   (counting from 0), and the character itself as second argument.val map : (char -> char) -> string -> stringString.map f s applies function f in turn to all
   the characters of s and stores the results in a new string that
   is returned.val trim : string -> string' ',
   '\012', '\n', '\r', and '\t'.  If there is no leading nor
   trailing whitespace character in the argument, return the original
   string itself, not a copy.val escaped : string -> stringval index : string -> char -> intString.index s c returns the character number of the first
   occurrence of character c in string s.
   Raise Not_found if c does not occur in s.
val rindex : string -> char -> intString.rindex s c returns the character number of the last
   occurrence of character c in string s.
   Raise Not_found if c does not occur in s.
val index_from : string -> int -> char -> intString.index_from s i c returns the character number of the
   first occurrence of character c in string s after position i.
   String.index s c is equivalent to String.index_from s 0 c.
   Raise Invalid_argument if i is not a valid position in s.
   Raise Not_found if c does not occur in s after position i.
val rindex_from : string -> int -> char -> intString.rindex_from s i c returns the character number of the
   last occurrence of character c in string s before position i+1.
   String.rindex s c is equivalent to
   String.rindex_from s (String.length s - 1) c.
   Raise Invalid_argument if i+1 is not a valid position in s.
   Raise Not_found if c does not occur in s before position i+1.
val contains : string -> char -> boolString.contains s c tests if character c
   appears in the string s.val contains_from : string -> int -> char -> boolString.contains_from s start c tests if character c
   appears in s after position start.
   String.contains s c is equivalent to
   String.contains_from s 0 c.
   Raise Invalid_argument if start is not a valid position in s.
val rcontains_from : string -> int -> char -> boolString.rcontains_from s stop c tests if character c
   appears in s before position stop+1.
   Raise Invalid_argument if stop < 0 or stop+1 is not a valid
   position in s.
val uppercase : string -> stringval lowercase : string -> stringval capitalize : string -> stringval uncapitalize : string -> stringtypet =string
val compare : t -> t -> int