code: public static Set<ProteinModification> getByComponent(final Component comp1,
  			final Component... comps) {
  		lazyInit();
  		Set<ProteinModification> mods = byComponent.get(comp1);
  		if (mods==null) {
  			return Collections.emptySet();
  		}
  
  		if (comps.length==0) {
  			return Collections.unmodifiableSet(mods);
  		} else {
  			Set<ProteinModification> ret = new HashSet<ProteinModification>(mods);
  			for (Component comp:comps) {
  				mods = byComponent.get(comp);
  				if (mods==null) {
  					return Collections.emptySet();
  				} else {
  					ret.retainAll(mods);
  				}
  			}
  
  			return ret;
  		}
  	}
docs: Get ProteinModifications that involves one or more components.
  @param comp1 a {@link Component}.
  @param comps other {@link Component}s.
  @return a set of ProteinModifications that involves all the components.