Actually, GUS does provide a filter for RPL-based expression slots. The filters are generally written to reflect implementation-level properties in order to provide an unambiguous result.

There are two distinct filters involved in Expression-related Slots. The "Slot Type" filter enumerates the ACTUAL Slot Types, as internally represented. "Series Slots with RPL Expression" are Series Slots -- not Expression Slots.

The "Has RPL Expression" filter can be used to find both Series Slots and Scalar Slots "with Rpl Expressions".

It would be less confusing if we displayed "Expression (Tcl Type)" or "Expression (Old)" as the "Slot Type" filter value (i.e. instead of just "Expression"). This change can, I believe, be made easily with a trivial change to method [Sim] RootFilter_SlotType::initSlotTypeRecs(). (Testing will reveal whether or not the labels are "magic" strings, but I am almost certain that they are not).

See additional screenshots, below.