User blog comment:Syst3ms/YAUD/@comment-30754445-20180810132619/@comment-35392788-20180812203519

Of course a regular OCF can handle it in the subscript, but here, not only does is have to be of a certain form, but the collapsing does not care in the slightest about the value of \(\psi_M(x)\), but only about \(x\). Correct me if I'm wrong, but the value of the subscript actually matters in the collapse.