Modal programs are represented as a series of rules, formatted as tokens delimited by brackets and parentheses, applied to a given tree which gets continually modified until no rules match any given part of the tree.

<> (?x dup) (?x ?x)
<> (?x ?y swap) (?y ?x)
<> ( ?x pop) ()

.. (1 2 3) (4 5 6) swap pop dup
01 (4 5 6) (1 2 3) pop dup
02 (4 5 6) dup
00 (4 5 6) (4 5 6)

<> (if (#t) ?b) (?b)
<> (if (#f) ?b) ()

.. (if (eq bat bat) reached!)
02 (if (#t) reached!)
00 (reached!)