Fixed an error in the maintenance of predicates defined by linear recursion rules. If there are any changes to the grouping predicates that appear in the body of a previously-defined linear recursion P2P rule, the rule is now re-evaluated. The previous (incorrect) behaviour was to silently ignore such changes.

The string:min and string:max predicates had been implemented for some time but an oversight in the implementation made them inaccessible. This has been fixed.