code: protected boolean semanticallyEquivalent(BioPAXElement other)
  	{
  		final xref anXref = (xref) other;
  
  		return
  			(DB != null ?
  				DB.equalsIgnoreCase(anXref.getDB()) :
  				anXref.getDB() == null)
  				&&
  				(ID != null ?
  					ID.equalsIgnoreCase(anXref.getID()) :
  					anXref.getID() == null)
  				&&
  				(DB_VERSION != null ?
  					DB_VERSION.equalsIgnoreCase(anXref.getDB_VERSION()) :
  					anXref.getDB_VERSION() == null)
  				&&
  				(ID_VERSION != null ?
  					ID_VERSION.equalsIgnoreCase(anXref.getID_VERSION()) :
  					anXref.getID_VERSION() != null);
  	}
docs: --------------------- Interface BioPAXElement ---------------------