Toyed and actually purchased PR a few years back but since my PC crashed and I’ve not used it. Node editing, imo, is the future and have ran across various specialy programs that implemented this as well as used node editing programs even earlier (Mathmap has this ability). GIMP 3.x supposedly has this ability but don’t believe they ever pursued this. Node editing makes creating personalized filters child’s play, imo. In a past life, used Engineering tools that also used node editing for programming workflow. Hopefully, they will allow this to come to fruition in GIMP 3.x. 
edit:
posted something a few years ago that used PR (link below).