def METHOD_NAME(self, row):
    'open copy of source sheet with rows that are grouped in current row'
    if row.sourcerows:
        vs = copy(self.source)
        vs.names = vs.names + [vd.valueNames(row.discrete_keys, row.numeric_key)]
        vs.rows=copy(row.sourcerows)
        return vs
    vd.warning("no source rows")