So, if you have a huge 'slew' limit, then [slop~] acts as a regular lowpass filter, and it can be implemented in [fexpr~] as
[fexpr~ $y + inc * ($x - $y)] (where 'inc' is the freq input in radians per sample).
I get this, what I don;t get is how it interplays with the other parameters when you have a slew limit of 0 or low...