public class HtmlElementCollection : System.MarshalByRefObject 

public static int Execute(Instance instance, IZennoPosterProjectModel project)
{
    // clear cookie
    instance.ClearCookie();
    
    // go to lessons.zennolab.com
    Tab tab = instance.MainTab;
    if ((tab.IsVoid) || (tab.IsNull)) return -1;
    if (tab.IsBusy) tab.WaitDownloading();
    tab.Navigate("lessons.zennolab.com");
    if (tab.IsBusy) tab.WaitDownloading();
    
    // find elements by tag
    HtmlElementCollection heCol = tab.FindElementsByTags("input:checkbox");
    
    // if collection is invalid
    if ((heCol.IsVoid) || (heCol.ErrorDetected)) return -1;
    
    HtmlElement he = null;
    
    for (int i = 0; i < heCol.Count; i++) he = heCol.Elements[i];
    
    // get index of html element in collection
    int index = heCol.IndexOf(he);
    
    return 0;
}

public static function Execute($instance, $project)
{
    // clear cookie
    $instance->ClearCookie();
    
    // go to lessons.zennolab.com
    $tab = $instance->MainTab;
    if (($tab->IsVoid) || ($tab->IsNull)) return -1;
    if ($tab->IsBusy) $tab->WaitDownloading();
    $tab->Navigate("lessons.zennolab.com");
    if ($tab->IsBusy) $tab->WaitDownloading();
    
    // find elements by tag
    $heCol = $tab->FindElementsByTags("input:checkbox");
    
    // if collection is invalid
    if (($heCol->IsVoid) || ($heCol->ErrorDetected)) return -1;
    
    for ($i = 0; $i < $heCol->Count; $i++) $he = $heCol->Elements[$i];
    
    // get index of html element in collection
    $index = $heCol->IndexOf($he);
    
    return 0;
}