code: public static void unregister(ProteinModification modification) {
  		if (modification==null) throw new IllegalArgumentException("modification == null!");
  
  		registry.remove(modification);
  
  		byId.remove(modification.getId());
  
  		Set<ProteinModification> mods;
  
  		mods = byResidId.get(modification.getResidId());
  		if (mods!=null) mods.remove(modification);
  
  		mods = byPsimodId.get(modification.getPsimodId());
  		if (mods!=null) mods.remove(modification);
  
  		mods = byPdbccId.get(modification.getPdbccId());
  		if (mods!=null) mods.remove(modification);
  
  		for (String keyword : modification.getKeywords()) {
  			mods = byKeyword.get(keyword);
  			if (mods!=null) mods.remove(modification);
  		}
  
  		ModificationCondition condition = modification.getCondition();
  		List<Component> comps = condition.getComponents();
  		for (Component comp : comps) {
  			mods = byComponent.get(comp);
  			if (mods!=null) mods.remove(modification);
  		}
  
  		byCategory.get(modification.getCategory()).remove(modification);
  		byOccurrenceType.get(modification.getOccurrenceType()).remove(modification);
  	}
docs: Remove a modification from registry.
  @param mod