The dynamic rules mechanism can be used to enable a set of rules.

  TriggerRules = 
     ?Context(Bla, _);
      rules( A : ... -> ...   B : ... -> ... )

  traverse =
     rec x({| A, B : try(TriggerRules + A + B); all(x) |})

Rules A and B are only applied after a Context(Bla,...) has been encountered.

This is not always appropriate. Often the applicability of rules can be determined by the position in the tree.

-- EelcoVisser & Lennart Swart - 22 May 2002