ndEpisode(1.0, true); })
    } else {
      d3.select(e).on('click', function(){ core.endEpisode(-1.0); })
    }
  }

  return correctText;
}


var genProblem = function() {  
  // init: click-button
  var div = d3.select('#area');
  div.html(''); // clear previous problem, if any  

  // 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 && !must_make_button) {
   