text: body.html('');
  title = 'JavaScript Expressions';
  d3.select('title')
    .html(title);
  
  body.append('h1')
    .html(title)
  
  body.append('p')
    .html('An expression is any valid unit of code that resolves to a value.');
  
  output = body.append('div')
    .style('border', '1px solid #CCC')
    .style('margin', '0.5em')
    .style('padding', '0.5em');
  
  next_button = body.append('button')
    .html('next >')
    .on('click', next);
  
  expression_list = [
  '1',
  '1 + 1',
  'x',
  'x = 1',
  'x'
  ];
  
  function next () {
    expression = expression_list.shift();
    if(expression_list.length < 1) { 
      next_button.style('display', 'none');
    };
    output.append('pre')
      .style('color', 'green')
      .text('expression > ' + expression);
    setTimeout(function () {
      try {
        value = eval(expression);
        output.append('pre')
          .style('color', 'blue')
          .text('     value > ' + value);
      }
      catch (e) {
        output.append('pre')
          .style('color', 'red')
          .text('     error > ' + e);
      }
    }, 1000);
  }
  