Write a package body for the list abstraction of Figure 10.2.

Figure 10.2

generic type item is private; default_value : item; Ada supports both type -- and value generic parameters -- package g_list is list_err : exception; type list_node is tagged private; 'tagged' means extendable; 'private' means opaque -- type list_node_ptr is access all list_node; 'all' means that this can point at 'aliased' nonheap data procedure initialize(self : access list_node; v : item := default_value); 'val' will get default value if second parameter is not provided -- --- procedure finalize(self : access list_node); function get_val (self : access list_node) return item; function predecessor (self : access list_node) return list_node_ptr; function successor (self : access list_node) return list_node_ptr; function singleton(self : access list_node) return boolean; procedure insert_before (self : access list_node; new_node : list_node_ptr); procedure remove (self : access list_node); type list is tagged private; type list_ptr is access all list; procedure initialize(self : access list); procedure finalize(self : access list); function empty(self : access list) return boolean; function head (self : access list) return list_node_ptr; procedure append (self : access list; new_node : list_node_ptr); private type list_node is tagged record prev, next, head_node : list_node_ptr; val : item; end record3; type list is tagged record head_node : aliased list_node; -- 'aliased' means that an 'all' pointer can refer to this end record; end g_list; ... package body g_list is definitions of subroutines -- ... end g_list;