@lazyattr
    def index_variables(self):
        """The columns participating in the index expression."""

        idxexprs = self.index_expressions
        idxvars = []
        for expr in idxexprs:
            idxvar = expr[0]
            if idxvar not in idxvars:
                idxvars.append(idxvar)
        return frozenset(idxvars)
dataset/ETHPy150Open PyTables/PyTables/tables/conditions.py/CompiledCondition.index_variables/original