var sum = dispatcher([
  function() { return 0 },
  function(x) { return x },
  function(x, y) { return x + y }
], function(x, y, z) {
  return Array.prototype.slice.call(arguments, 1).reduce(function(x, y) {
    return sum(x, y)
  }, x)
})

sum()         // => 0
sum(2)        // => 2
sum(2, 3)     // => 5
sum(2, 3, 4)  // => 9