'Compute our stored expression string with numexpr.'

def _compute(self, arrays, dates, assets, mask):

    out = full(mask.shape, self.missing_value, dtype=self.dtype)
    numexpr.evaluate(self._expr, local_dict={('x_%d' % idx): array for (idx, array) in enumerate(arrays)}, global_dict={'inf': inf}, out=out)
    return out
