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(* Stacks *)
(* This modl 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 clear : 'a t -> unit
(* Discard all elements from a stack. *)
val length: 'a t -> int
(* Return the number of elements in a stack. *)
val iter: ('a -> 'b) -> 'a t -> unit
(* [iter f s] applies [f] in turn to all elements of [s], from the
element at the top of the stack to the element at the
bottom of the stack. The stack itself is unchanged. *)
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