@classmethod
def xform(cls, column_headers, lines):
    """
        Transform column values and yield just the line info.

        :param column_headers: the column headers
        :type column_headers: list of str
        :param lines: information about each line
        :type lines: dict of str * str
        """
    key = column_headers[0]
    for line in lines:
        line_info = line['node']
        line_info[key] = cls.calculate_prefix(line) + line_info[key]
        yield line_info
