c: conversion
d: decomposed rt-reduction
e: decomposed rt-conversion
-g: generic rt-transition
+g: counted rt-transition (generic)
q: restricted reduction
r: reduction
s: substitution
u: supclosure
w: reserved for generic pointwise extension
-x: rt-reduction
+x: uncounted rt-transition (extended)
y: rt-substitution
- forth letter (if present)