function array_reduce( a_input, callback ) {	// Iteratively reduce the array to a single value using a callback function
	// 
	// +   original by: Alfonso Jiménez (http://www.alfonsojimenez.com)

	var lon = a_input.length;
	var res = 0;
	var tmp = new Array();

	for(i = 0; i < lon; i += 2) {
		tmp[0] = a_input[i];
		if(a_input[i+1]){
			tmp[1] = a_input[i+1];
		} else {
			tmp[1] = 0;
		}
		res += callback.apply(null, tmp);
		tmp = new Array();
	}

	return res;
}