/** Sets what should happen to values of this property if the owning peanut is deleted 
	* If not set, nothing happens. If set to 'd' the delete will cascade to the property values.
	* If set to 'c' the the owning peanut can only be deleted if the property has no values.
	* If set to 'v' (verify) the user interface should verify the recursive delete with the user first.
	* 	(in versions earlier then 2.0 this was not implemented)
	*/
	function setOnDelete($letter) {
		if ($letter !== null && strPos('dvc', $letter) === false)
			trigger_error('onDelete letter not recognised', E_USER_WARNING);
		$this->onDelete = $letter;
	}