- third letter
-b: "big tree" reduction
+b: (q)rst-reduction
c: conversion
-d: decomposed extended reduction
-e: decomposed extended conversion
+d: decomposed rt-reduction
+e: decomposed rt-conversion
q: restricted reduction
r: reduction
s: substitution
u: supclosure
w: reserved for generic pointwise extension
-x: extended reduction
-y: extended substitution
+x: rt-reduction
+y: rt-substitution
- forth letter (if present)