function change_image (element, url)
{
 if (document.getElementById (element))
    {
      document.getElementById (element).src = url;
    }
}

function set_class (element, className)
{
  if (document.getElementById (element))
    {
      document.getElementById (element).className = className;
    }
}

function set_class_all (prefix, className, max)
{
  max = max ? max : 100;
  for (i=0; i<max; i++)
    {
      if (document.getElementById (prefix+i))
        {
          document.getElementById (prefix+i).className = className;
        }
      else
        break;
    }
}

function show (element)
{
  if (document.getElementById (element))
    {
      document.getElementById (element).style.display = 'block';
    }
}

function taofocus (element)
{
  if (document.getElementById (element))
    {
      document.getElementById (element).focus ();
    }
}

function hide (element)
{
  if (document.getElementById (element))
    {
      document.getElementById (element).style.display = 'none';
    }
}

function hide_all (prefix, max)
{
  max = max ? max : 100;
  for (i=0; i < 20; i++)
    {
      if (document.getElementById (prefix+i))
        {
          document.getElementById (prefix+i).style.display = 'none';
        }
//      else        break;
    }
}