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What is currentFunc, though? Normally that is a parser class instance variable set to the currently matched function by an action in the parser grammar. For constructs that can nest such as classes in some languages, the "current class" pointer needs to be treated like a stack. ANTLR has dynamically scoped very rules for this purpose; perhaps we could do something similar.

method:
  scope { FunctionSymbol currentFunc; }
  blah blah blah

The result of a pattern match could be an action or perhaps even just a context with no pattern:

method:
  {System.out.println("heh, I just entered a method")} // always exec in method context
  type ID {currentFunc = currentScope.resolve($ID.text);} // exec this but referredrefer to ID

We might also need to be able to specify a predicate on token references:

expr:
  ID<"null"> -> "nil" // test for null keyword as ID
  ^('*' INT<{is power of 2}> expr) -> (lg2={$INT.int}) "<expr> << <lg2>"

Rewrites for tree grammars

There is no reason we could not do rewrite patterns on trees. The left would always be a tree pattern but the right could be a tree pattern or template.

expr:
  ^('+' expr expr) -> add(blah blah blah) // rewrite with template ref
  INT -> "0" // differentiate any integer to constant "0"

To rewrite to another tree:

expr:
  ^('+' INT<"0"> expr) -> expr
  ^('*' INT<"1"> expr) -> expr