code: def is_cell_separator(self, cursor=None, block=None):
          
          assert cursor is not None or block is not None
          if cursor is not None:
              cursor0 = QTextCursor(cursor)
              cursor0.select(QTextCursor.BlockUnderCursor)
              text = to_text_string(cursor0.selectedText())
          else:
              text = to_text_string(block.text())
          if self.cell_separators is None:
              return False
          else:
              return text.lstrip().startswith(self.cell_separators)
docs: Return True if cursor (or text block) is on a block separator