anchor.diff:       (filter #'fuzzy-match
                :type function
                :documentation
  -             "Takes `input' and filters the suggestions.")
  +             "Takes `input' and a `suggestion' and return a new suggestion, or
  +nil if the suggestion is discarded.")
   
        (filter-preprocessor #'delete-inexact-matches
                             :type (or null function)
  
positive.code: ;; We must eval the class at read-time because `make-source' is generated using
  ;; the initargs of the class.
  (sera:eval-always
    (define-class prompter-source ()
      ((name ""
             :documentation
             "Name which can be used to differentiate sources from one
  
negative.code: non-nil.")
  
       (must-match-p :always
                     :type (or must-match-choices null)
                     :documentation
                     "Control what to do when input does not match anything.
  - `:always': Reject input.
  