Event.observe(window, 'load', function() {
  $$('.fpsection').each(function(sec){
    var s = sec.getAttribute('id').replace('^section_', '');
    sec.select('a').each(function(anc) {
      Event.observe(anc, 'mouseover', function(ev) { setOp(s, 1); });
      Event.observe(anc, 'mouseout', function(ev) { setOp(s, 0); });
    });
  });
});

function setOp(which, opaque){
  if(opaque) {
    $(which).removeClassName('opacity1');
    $(which).addClassName('opacity2');
  } else {
    $(which).removeClassName('opacity2');
    $(which).addClassName('opacity1');
  }
}