an extension point for specializations of the positional rest tag name.
Stripe-skipping is not allowed for positional rest variables.
A positional rest variable used in <Atom>s, <Expr>s and <Plex>s.
Note that <Plex>s are generated, so <repo>s are only available above the Datalog sublanguage.
See http://www.ruleml.org/0.9/glossary/#gloss-repo
The content model of a positional rest variable is a single term.
terms allowed in positional rest variables include sequence markers.
Sequence markers are represented as ordinary variables.