unremovable text tags

//SOLVED, see: http://comments.gmane.org/gmane.comp.gnome.lib.gtk%2B.devel.apps/32646
using Gtk;

//compile with: valac --pkg gtk+-3.0 textTagTest.vala
void main(string[] args)
{
    Gtk.init(ref args);
    TextView textView;
    Gtk.TextBuffer buffer;
    Gtk.TextIter startIter, currentIter, lastIter, endIter;
    Gtk.TextTag tag;
    Gtk.TextMark lastMark;
    Window win;
    string tagName = "";

    textView = new TextView();
    buffer = textView.buffer;

    buffer.get_start_iter(out startIter);//get the iterator at the beginning.
    lastMark = buffer.create_mark("lastMark", startIter, true);//set a mark at the beginning.

    int loopCounter = 0;

    //The following loop adds three text tags with a unique tag-name to the text buffer.
    //Then it attempts to delete them (buffer.remove_all_tags(...) ).
    //In the next iteration I expect to be able to use the same tag-names as they should have been deleted.
    //But I get errors saying those tag names already exist in the buffer.
    while (loopCounter++ < 2)
    {
        for(int tagCounter = 0; tagCounter < 3; tagCounter++)
        {
            tagName = "tag_" + tagCounter.to_string();//create a unique tagname.
            buffer.set_text(tagName.to_string()+"\n", -1);//write "tag_<num>" into buffer
            buffer.get_iter_at_mark(out lastIter, lastMark);//where was the cursor placed before we wrote text?
            buffer.get_iter_at_offset(out currentIter, buffer.cursor_position);//where is the cursor placed now?
            tag = buffer.create_tag(tagName);//create a tag named "tag_<num>"
            buffer.apply_tag(tag, lastIter, currentIter);//apply the tag from the beginning to the end of the written text.
            buffer.delete_mark(lastMark);
            lastMark = buffer.create_mark("lastMark", currentIter, true);//upcoming text to be placed at the current cursor pos.
            Thread.usleep(1000000);//delay to monitor on stdout.
        }
        //when done, delete everything.
        buffer.get_start_iter(out startIter);//get the very first text iterator
        buffer.get_end_iter(out endIter);//get the very last iterator (redundant, but to be sure).
        buffer.remove_all_tags(startIter, endIter);//remove all created tags.
        buffer.delete(ref startIter, ref endIter);//remove all written text.
        //we should be safe to start over with the tag names "tag_0", "tag_1", "tag_2" again.
    }

    win = new Window();
    win.title = "TextTags";
    win.border_width = 5;
    win.window_position = WindowPosition.CENTER;
    win.set_default_size(655, 480);
    win.add(textView);
    win.show_all();

    Gtk.main();
}