anchor.diff:      !! Returns true if the two keys are equal.
       !!
       !! Allowing a key to be an integer or a character string
  -    !! (can be case sensitive or not).
  +    !! (can be case sensitive or not), or alternately, a user-defined
  +    !! [[key_class]].
   
       implicit none
   
  
positive.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 or string.
  
      module linked_list_module
  
  
negative.code: 
      type(node),pointer :: p
  
      !only allowing integer, string, or key_class 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 is (key_class)
          !ok
      class default
          error stop 'Error: key must be an integer, character string, or key_class.'
      end select
  
      ! if the node is already there, then remove it
  