Often, you want your parser to parse your text and return a parse
tree that then can be used for further processing.
One way of doing it is to make actions at each production rule
that return the appropriate datastructure.
Another way is the autotree directive.
<autotree> should preceed the first rule. If <autotree>
is present, the parser generator will add an action to each production
that doesn't end in an action already.
For non-terminal productions, the added action is: