I wrote:
> I should better revert yesterday's patch, and instead,
> in the table show the guaranteed average performance
>   gl_list_get_first
>   gl_list_get_last
>   gl_list_set_first
>   gl_list_set_last
>   gl_list_remove_first
>   gl_list_remove_last
> where these 6 functions are defined globally, not separately for each
> implementation.

Done through the two attached patches.

2020-05-02  Bruno Haible  <bruno@clisp.org>

	list: Add get_first, get_last, set_first, set_last operations.
	* lib/gl_list.h (gl_list_get_first, gl_list_get_last,
	gl_list_nx_set_first, gl_list_nx_set_last): New functions.
	* lib/gl_xlist.h (gl_list_set_first, gl_list_set_last): New functions.

2020-05-02  Bruno Haible  <bruno@clisp.org>

	list: Remove redundant code for remove_first and remove_last operations.
	* lib/gl_list.h (struct gl_list_implementation): Remove fields
	remove_first, remove_last.
	(gl_list_remove_first, gl_list_remove_last): Implement in a generic way.
	* lib/gl_array_list.c: Revert last change.
	* lib/gl_carray_list.c: Likewise.
	* lib/gl_anylinked_list2.h: Likewise.
	* lib/gl_linked_list.c: Likewise.
	* lib/gl_linkedhash_list.c: Likewise.
	* lib/gl_anytree_list2.h: Likewise.
	* lib/gl_avltree_list.c: Likewise.
	* lib/gl_avltreehash_list.c: Likewise.
	* lib/gl_rbtree_list.c: Likewise.
	* lib/gl_rbtreehash_list.c: Likewise.
	* lib/gl_sublist.c: Likewise.