function change1a() {
 if(document.getElementById("bgr1").className == "nvis1")
  document.getElementById("bgr1").className = "on1";
}

function change1b() {
 if(document.getElementById("bgr1").className == "on1")
  document.getElementById("bgr1").className = "nvis1";
}

function change2a() {
 if(document.getElementById("bgr2").className == "nvis2")
  document.getElementById("bgr2").className = "on2";
}

function change2b() {
 if(document.getElementById("bgr2").className == "on2")
  document.getElementById("bgr2").className = "nvis2";
}

function change3a() {
 if(document.getElementById("bgr3").className == "nvis3")
  document.getElementById("bgr3").className = "on3";
}

function change3b() {
 if(document.getElementById("bgr3").className == "on3")
  document.getElementById("bgr3").className = "nvis3";
}