Missing warning

[Norman . Ulbrich]

When an action is specified for a rule, and there is no assignment to $$,
there should be a warning.
Example:

...
argumentlist: argument argumentlist { newList($1, $2); /* *** */}
            | /*empty*/ {$$=NULL;}
            ;
...

In the line with the three stars comment "$$=" is missing: the return value
of newList shall become the value of the argumentlist.
This mistake can easily happen.

My suggestion is to print a warning if a semantic action but no "$$=" is
found. The default action "$$=$1" might be wrong in many cases.

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