anchor.diff:  
           private
   
  -        procedure,public :: add                     !! add a pointer item to the list
  +        procedure,public :: add_pointer             !! add a pointer item to the list
           procedure,public :: add_clone               !! add a non-pointer item to the list
           procedure,public :: get => get_data         !! get a pointer to an item in the list
           procedure,public :: destroy => destroy_list !! destroy the list and
  
positive.code: 
      type(node),pointer :: p
  
      !only allowing integer or string keys:
      select type (key)
      type is (integer)
          !ok
      type is (character(len=*))
          if (len_trim(key)<1) error stop 'Error: key must be nonblank.'
      class default
          error stop 'Error: key must be an integer or character string.'
      end select
  
      ! if the node is already there, then remove it
  
negative.code: ! A generic list.
  !
  ! It uses an unlimited polymorphic `class(*)` pointer variable to allow it
  ! to contain any type of data. The `key` can be an integer, string, or
  ! any user-defined [[key_class]].
  
      module linked_list_module
  
  