# first param is ref to hash, other params are valid keys
# if invalid key is found, return "invalid key:$key"
sub check_keys($@) {
    my @l1 = sort(keys(%{ shift(@_) }));
    my @l2 = sort( @_ );

    while (@l1) {
        return "invalid key:$l1[0]" if (not @l2) or ($l2[0] gt
        $l1[0]);
        shift @l1 while @l1 and @l2 and ($l1[0] eq $l2[0]);
        shift @l2 while @l1 and @l2 and ($l1[0] gt $l2[0]);
    }
    return '';
}