Awesome partial application function for JavaScript

papply.js

var __ = {}
function papply(func, givenArguments) {
  var indexOf = givenArguments.indexOf(__)
  var givenArgsSize = givenArguments.length
  var requiredArgsSize = func.length
  if(givenArgsSize >= requiredArgsSize && (indexOf < 0 || indexOf >= requiredArgsSize))
    return func.apply(func, givenArguments)
  else
    return function() {
      return (function(givenArguments, remainingArguments) {
        for(var i = 0; i < givenArguments.length; i++)
          if(givenArguments[i] === __) givenArguments[i] = remainingArguments.shift()
        return papply(func, givenArguments.concat(remainingArguments))
      })(givenArguments.slice(0), Array.prototype.slice.call(arguments))
    }
}

Function.prototype.papply = function() {
  return papply(this, Array.prototype.slice.call(arguments))
}

//EXAMPLE
function sum(a, b, c, d) { return a + b + c + d}

//All following examples return 'abcd'
sum.papply('a', 'b', 'c', 'd')
sum.papply('a', 'b')('c', 'd')
var ab = sum.papply('a', 'b')
ab('c','d')
ab('c','d')
ab('c')('d')
sum.papply('a', __, 'c', __)('b','d') 
sum.papply('a', __, 'c', __)('b')('d')