Singly linked list implementation

sllist.adb

with Ada.Strings.Unbounded; use Ada.Strings.Unbounded; 
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Text_IO.Unbounded_IO; use Ada.Text_IO.Unbounded_IO;
procedure SLList is 
   function "+" (Source : in String)
                return Unbounded_String renames To_Unbounded_String;
   
   type Node;
   type Node_Ref is access Node;
   type Node is record 
      S: Unbounded_String;
      Next: Node_Ref;
   end record;
   
   procedure Traverse (First : Node_Ref) is 
      P : Node_Ref := First;
   begin
      while P /= null loop 
         Put_Line (P.S);
         P := P.Next;
      end loop;
   end Traverse;
   
   procedure Insert (First : in out Node_Ref; New_Node : Node_Ref) is 
      P : Node_Ref;
   begin
      if First = null or else New_Node.S < First.S then 
         New_Node.Next := First;
         First := New_Node;
      else
         P := First;
         while P.Next /= null and then New_Node.S > P.Next.S loop 
            P := P.Next;
         end loop;
         New_Node.Next := P.Next;
         p.Next := New_Node;
      end if;
   end Insert;
   
   --  Build the list manually.
   S3 : Node_Ref := new Node'(+"x", null);
   S2 : Node_Ref := new Node'(+"m", S3);
   S1 : Node_Ref := new Node'(+"a", S2);
   
   -- Save a reference to the first item in the list.
   First : Node_Ref := S1;
   
   -- Create some other nodes to use later.
   S4 : Node_Ref := new Node'(+"o", null);
   S5 : Node_Ref := new Node'(+"z", null);
   S6 : Node_Ref := new Node'(+"h", null);

begin 
   Put_Line (+"This is the Singly Linked List program.");
   
   Traverse (First);
   New_Line;
   
   Insert (First, S4);
   Insert (First, S5);
   Insert (First, S6);
   
   Traverse (First);
end SLList;