anchor.diff:  (defvar *empty-iterator*
     (make-iterator ()
       (stop)))
  -
  -(defun completion-suggestions-for-tokens (tokens cursor-point)
  -  (declare (ignore tokens cursor-point))
  -  *empty-iterator*)
   
   (defun iterator-without-duplicates (iter)
     (let ((seen-values (fset:empty-set)))
  
positive.code:       (vector-push-extend error all-errors))
      (setf error (make-instance 'shcl/core/parser:choice :errors all-errors))
      (iterator-without-duplicates
       (concatenate-iterators
        (map-iterator (expected-types-for-parse-error error sigil-token)
                      'simple-expansions-for-type)))))
  
  (defun completion-suggestions-for-input (input-text cursor-point readtable)
    "Compute possible completions.
  
negative.code:           (setf end-found t)
            (return))))
      (if end-found
          (completion-suggestions-for-tokens tokens (vector-pop tokens) cursor-point)
          (completion-suggestions-for-start-of-new-token tokens))))
  