union_expr
RESULT = union_expr(LHS, RHS); \
$$ = union_expr($1, union_expr($3, $5));
$$ = union_expr($1, $3);
$$ = union_expr($1, $3);
$$ = union_expr($1, $3);
$$ = union_expr($1, $3);
$$ = union_expr($1, $3);
$$ = union_expr($3, $5);
$$ = union_expr($3, $5);
$$ = union_expr($3, $5);