- Selection rules are UBG rules that match the stack and select a
grammar rule to apply.
- They can apply a limited degree into the stack. This gives
- They have preference levels.
- General rules have lower preference values.
- General selection rules do most of the work, but specific selection
rules can catch the exceptions.
- Preference levels can be based on the input, either by a constant
from the lexemes or unification of constants.
- When ambiguity occurs, semantics usually resolves