Selection Rules
- are UBG rules that match the stack and select a grammar rule
to apply.
- can apply a limited degree into the stack. This gives look-ahead.
- have preference levels.
- Multiple rules can be selected.
- The grammar with the highest selection preference level is applied
first.
- This is repeated until a grammar rule is applied successfully.
- If no grammar rule is applied successfully, the next token is
pushed.
- Preference levels can be based on the input, either by a constant
from the lexemes or unification of constants.
- A push can be selected to delay a decision.
