;optional
                (progress-bar (or null cl-progress-bar.progress:progress-bar)
                              *progress-bar*
                              &quot;Instance of progress-bar. Usually should be
                               left with default (namely: *progress-bar*).&quot;))
  -&gt; null
  (:doc &quot;Notify progress bar about step completion.&quot;
   :thread-safety &quot;This function is thread safe.&quot;)
  
  (when progress-bar
    (bt:with-lock-held ((cl-progress-bar.progress:mutex