 
  var dialogDiv = $('#dialog');
  resetUI(dialogDiv);

  // init: click-button
  var div = d3.select('#area');
  div.html(''); // clear previous problem, if any
  
  // main: click link  
  var correctText = createLinks(div);
  var any = core.getOpt(core.QueryString, 'any', false); // click Any link?

  // main: click-button
  var num = 6;
  var elements = [];
  for(var i=0;i<num;i++) {
    var u = core.randf(0,1);
    var must_make_button = (i === num-1) && elements.length === 0;

    if(u < 0.33 && !mus