(* Stack.sig *) (* This module implements stacks (LIFOs), with in-place modification. *) type 'a t;        (* The type of stacks containing elements of type ['a]. *) exception Empty;        (* Raised when [pop] is applied to an empty stack. *) val new: unit -> '_a t;      (* Return a new stack, initially empty. *)val push: 'a -> 'a t -> unit;      (* [push x s] adds the element [x] at the top of stack [s]. *)val pop: 'a t -> 'a;      (* [pop s] removes and returns the topmost element in stack [s],         or raises [Empty] if the stack is empty. *)val peek: 'a t -> 'a;      (* [pop s] returns the topmost element in stack [s],         without removing it, or raises [Empty] if the stack is empty. *)val update: 'a -> 'a t -> unit;      (* [update x s] replaces the top element of stack [s] with [x]. *)val null: 'a t -> bool;      (* [null s] returns true iff stack [s] is empty. *)val clear : 'a t -> unit;      (* Discard all elements from a stack. *)