12. Memoizing Rules That Read Variables

• Rules that read variables, directly or through callees, were never memoized, because their results depend on the environment. Context-sensitive grammars (indentation-based ones in particular) could then take exponential time, and the Python example had to be written in a parse-once-then-fold style to avoid it.
• The memo key now includes the values of the variables the rule may read (rule.vars, computed statically and sorted); definitions made inside a rule are undone when it returns, so these values determine the result. Generated parsers do the same (the generator emits each rule's variable list).
• Effect: removes the exponential worst case for variable-reading rules; no change for grammars without variables.