// Precarga de imagenes
function preLoad(dir, pfx, num, let, tipo) {
  if (document.images) {
    eval (pfx + num + let + ' = new Image()');
    eval (pfx + num + let + '.src = "' + dir + '/' + pfx + num + let + tipo + '"');
  }
}

function chgMnu(num, let) {
  b = document.getElementById('btn' + num);
  f = document.getElementById('fot' + num);
  eval ('b.src = btn' + num + let + '.src');
  eval ('f.src = fot' + num + let + '.src');
}