Как работает данный полифилл?

(function() { 
  var el = document.documentElement; 
  if (!el.compareDocumentPosition && el.sourceIndex !== undefined) { 
 
    Element.prototype.compareDocumentPosition = function(other) { 
      return (this != other && this.contains(other) && 16) + 
        (this != other && other.contains(this) && 8) + 
        (this.sourceIndex >= 0 && other.sourceIndex >= 0 ? 
          (this.sourceIndex < other.sourceIndex && 4) + 
          (this.sourceIndex > other.sourceIndex && 2) : 1 
        ) + 0; 
    } 
  } 
}());