code: def find_comment_markers(cellsource):
      
      found = {}
      for line in cellsource.splitlines():
          line = line.strip()
          if line.startswith('#'):
              # print("Found comment in '{}'".format(line))
              comment = line.lstrip('#').strip()
              if comment in comment_markers:
                  # print("Found marker {}".format(comment))
                  marker = comment_markers[comment]
                  if not isinstance(marker, tuple):
                      # If not an explicit tuple ('option', True/False),
                      # imply ('option', True)
                      marker = (marker, True)
                  marker_type = marker[0]
                  if marker_type in found:
                      warnings.warn(
                          "Conflicting comment markers found, using the latest: "
                          " %s VS %s" %
                          (found[marker_type], comment))
                  found[marker_type] = comment
                  yield marker
docs: Look through the cell source for comments which affect nbval's behaviour
  
      Yield an iterable of ``(MARKER_TYPE, True)``.