an extension point for specializations of the slotted rest tag name.
Stripe-skipping is not allowed for slotted rest variables.
A slotted rest variable used in <Atom>s, <Expr>s and <Plex>s.
Note that <Plex>s are generated, so <resl>s are only available above the Datalog sublanguage.
See http://deliberation.ruleml.org/1.02/glossary/#gloss-resl
The content model of a slotted rest variable is a single term.
terms allowed in slotted rest variables.
Sequence markers are represented as ordinary variables.