concat_symbols
$$ = concat_symbols($1, $2);
$$ = concat_symbols($1, declare_member($4));
$$ = concat_symbols($1, declare_member($4));
$$ = concat_symbols($1, $3);
$$ = concat_symbols($1,
$$.first = concat_symbols($1.first, $3);
sym_t *concat_symbols(sym_t *, sym_t *);