cards notated '?1', '?2', etc. may be used in production rules. A
may be replaced with any terminal symbol, simple note, variable, bracket,
out-time object, synchronisation tag, time-pattern or performance parameter.
For example, a rule like
?2 ?3 ?1 --> (= ?1) (= ?2) ?3 * (: ?2) (: ?1)
"X Y Z X" as "(= X) (= Y) Z * (: Y) (: X)". (Symbol '*' is an homomorphism.)
is an old notation that may be replaced with '?n' for any value of n.