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TextFile

generic
    type Element is private;
    -- must be a pure value
    -- ie. no initialization or finalization is necessary
    -- = and := are equality and copy

    pragma Must_Be_Constrained (Yes => Element);

package List_Generic is

    type List is private;
    -- may generate garbage
    -- = and := operate on references
    -- "make" constructs lists with structural sharing

    -- constraint error is raised when nil i provided to any of
    --    first, rest, set_first, or set_rest

    function Make (X : Element; L : List) return List;

    function Nil                 return List;
    function Is_Empty (L : List) return Boolean;

    procedure Free (L : in out List);
    -- make L empty

    function  First     (L : List) return Element;
    function  Rest      (L : List) return List;
    procedure Set_Rest  (L : List; To_Be : List);
    procedure Set_First (L : List; To_Be : Element);

    function Length (L : List) return Natural;

    type Iterator is private;

    procedure Init  (Iter : out Iterator; L : List);
    procedure Next  (Iter : in out Iterator);
    function  Value (Iter : Iterator) return Element;
    function  Done  (Iter : Iterator) return Boolean;

private

    type Listdata;
    type List is access Listdata;
    -- variables of type list are initialized to null

    type Listdata is
        record
            First : Element;
            Rest  : List;
        end record;

    type Iterator is new List;

end List_Generic;