This makes checking of variables more strict: a variable can only
be used in the right -hand side of an assignment if it has been
assigned by an unconditional predecessor or if it has been matched
by a parent.
Things that wouldn't work anymore:
REF=X* {
FN=*V { X=Y }
Z=$X /* we can't be sure X=Y was taken */
}
FN=*R {
X=$REF /* no previous use of REF */
}
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