<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_dimension(cls, columns, dimension, dim_pos, values, vdim):
 """ Adding value dimensions not currently supported by iris interface. Adding key dimensions not possible on dense interfaces. """

        if not vdim:
            raise Exception("Cannot add key dimension to a dense representation.")
        raise NotImplementedError