A "rule" function gets as argument a list:

0 - a string to match 1 - an optional "continuation" 2 - an optional "flags" hashref 'capture'=>1 means 'return whatever matches'

it returns (or "yields"):

    undef - match failed

or a hash containing:

    state - a "continuation" or undef
    bool - an "assertion" (true/false)
    match - the "match" tree or undef
    tail - the string tail or undef
    capture - the tree of captured things
    abort - the match was stopped by a { return } or a fail(),
           and it should not backtrack or whatever

Continuations are used for backtracking.

A "ruleop" function gets some arguments and returns a "rule".