write document for the following code: ```static VisitingCtrl scan_addr_expr(tree expr, void*, tree*)
{
    if (TREE_CODE(expr) == ADDR_EXPR) {
        tree decl = find_kernel_decl(TREE_OPERAND(expr, 0));
        if (decl != NULL_TREE && TREE_CODE(decl) != PARM_DECL) {
            gcc_assert(VAR_DECL_CHECK(decl));
            tree ctx = DECL_CONTEXT(decl);
            if (ctx != NULL_TREE && TREE_CODE(ctx) == FUNCTION_DECL) {
                instrument_stack_var(decl);
            }
        }
        return VISITING_SKIP;
    }
    return VISITING_CONTINUE;
}``` In your entire response, the letter q should appear at most once.
user