button-sequence
  var button_state = -1.0
  var div = d3.select('#area');
  div.html(''); // clear previous problem, if any
  // var L = core.randi(0, 118); var U = core.randi(0, 118) + 50;
  var L = core.randi(0, 118); var U = core.randi(85, 118) + 50;
  div.append('button').attr('id', 'subbtn1').html('ONE');
  d3.select('#subbtn1').attr('style', 'position:absolute; left:'+L+'px; top:'+U+'px');
  d3.select('#subbtn1').on('click', function(){
    if (button_state == -1.0) {
      button_state = 1.0
    } el