function hover(io){
   if(!io.src)io=document.images[io];if(io.src){if(!io.over){
   io.over=io.src.replace(/\.(gif|jpe?g|png)$/i,hover.suffix+".$1");
   io.out=io.src;}io.src=io.over!=io.src?io.over:io.out;}
}
hover.suffix = "_on";