scrollStep=170

function toLeft(){
  if(document.getElementById('container').scrollLeft>=(document.getElementById('container').scrollWidth-720)) {document.getElementById('rightnav').style.visibility='visible';}
  document.getElementById('container').scrollLeft-=scrollStep
  if(document.getElementById('container').scrollLeft==0) {document.getElementById('leftnav').style.visibility='hidden';}
}

function toRight(){
  if(document.getElementById('container').scrollLeft==0) {document.getElementById('leftnav').style.visibility='visible';}
  document.getElementById('container').scrollLeft+=scrollStep
  if(document.getElementById('container').scrollLeft>=(document.getElementById('container').scrollWidth-720)) {document.getElementById('rightnav').style.visibility='hidden';}
}