// Stack type.
    type Stack() =

        // Private mutable list.
        let mutable stack = []

        // Pushing one element.
        member this.Push element =
            stack <- element::stack

        // Popping element.
        // If there are elements in stack we return
        // top element and rest of a stack.
        member this.Pop =
            match stack with
            | top::rest ->
                stack <- rest
                (top, stack)
            | [] -> failwith "Stack underflow"

        // If stack is empty.
        member this.IsEmpty =
            match stack with
            | [] ->
                true
            | _ ->
                false

        // Getting stack.
        member this.GetStack() =
            stack