'Return True if cursor (or text block) is on a block separator'

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)
