public function addField($name, $transformer = null, $pos = null) {
			if ($pos === null)
				$pos = sizeof($this->fields);

			$this->fields[$pos] = $name;

			if (is_callable($transformer))
				$this->transformers[$pos] = $transformer;
			else if ($transformer !== null)
				throw new InvalidArgumentException('The transformer provided for ' . $name . ' is not callable');

			return $this;
		}