code: protected static void replaceXrefsWithRelatedER(
  		Collection<BioPAXElement> elements)
  	{
  		Set<EntityReference> ers = new HashSet<EntityReference>();
  		Set<Xref> xrefs = new HashSet<Xref>();
  		for (BioPAXElement element : elements)
  		{
  			if (element instanceof Xref)
  			{
  				xrefs.add((Xref) element);
  				for (XReferrable able : ((Xref) element).getXrefOf())
  				{
  					if (able instanceof EntityReference)
  					{
  						ers.add((EntityReference) able);
  					}
  				}
  			}
  		}
  
  		elements.removeAll(xrefs);
  		for (EntityReference er : ers)
  		{
  			if (!elements.contains(er)) elements.add(er);
  		}
  	}
docs: Replaces Xref objects with the related EntityReference objects. This is required for the use
  case when user provides multiple xrefs that point to the same ER.
  @param elements elements to send to a query as source or target