pq-System
The pq-system Axioms and Rules
(from the book "Gödel, Escher, Bach: An Eternal Golden Braid" by Douglas Hofstadter)
Axiom definition: xp-qx- is an axiom, whenever x is composed of hyphens only.
Rule: if xpyqz is known to be a theorem,
(where x, y, and z are strings containing only hyphens)
then xpy-qz- is a theorem.