function goSub(num){
  switch(num){
  case 1:
    document.getElementById("sub1").style.display = "block";
    document.getElementById("sub2").style.display = "none";
    document.getElementById("sub3").style.display = "none";
    break;
  case 2:
    document.getElementById("sub1").style.display = "none";
    document.getElementById("sub2").style.display = "block";
    document.getElementById("sub3").style.display = "none";
    break;
  case 3:
    document.getElementById("sub1").style.display = "none";
    document.getElementById("sub2").style.display = "none";
    document.getElementById("sub3").style.display = "block";
    break;
  }
}