public function isFunctionUsed( $functions )
	{
		if ( is_string( $functions ) )
		{
			$functions = array( $functions );
		}

		$result = $this->traverseNodes( $this->exprTree, function( $node ) use( &$result, $functions )
		{
			// TODO Test the functions in this node and return false if any are found
			return true;
		} );

		return ! $result;
	}