mid-rule actions

[hz kto]

Hi, guys,

I have a question about mid-rule actions, here is the example from the bison 
documentation:

compound: '{' { prepare_for_local_variables (); }
          declarations statements '}'
        | '{' statements '}'
        ;

If we are parsing "declarations statements" from the first part of the rule and 
find out that we need to use just "statements" will 
prepare_for_local_variables() already be executed or will it run only if the 
"declarations statements" match?
If the former is the case, how do I undo the effects?

thanks in advance