(function(){
  var idx = 0;
  var bck = document.getElementById("bck");
  var fwd = document.getElementById("fwd");

  bck.addEventListener('click', function() { idx--; show(); });
  fwd.addEventListener('click', function() { idx++; show(); });
  document.addEventListener('keydown', function(e) {
    switch (e.which) {
      case 37:
        idx--;
        break;
      case 39:
        idx++;
    }
    show();
  });

  function show() {
    var poems = document.getElementsByClassName("poem");

    if (idx < 0) idx = poems.length-1;
    if (idx >= poems.length) idx = 0;

    for (var i = 0; i < poems.length; i++) poems[i].style.display = "none";

    poems[idx].style.display = "block";
  }
  show(idx);
})();