Added safety check to addBefore to ensure new element isn't already in a list. If this happens, most likely the method was called with the arguments out of order.
Added safety check to addBefore to ensure new element isn't already in a list.
If this happens, most likely the method was called with the arguments out of order.

etomica/etomica,ajschult/etomica,ajschult/etomica,etomica/etomica,etomica/etomica,ajschult/etomica