code: protected boolean isContained(T instance) {
  		for(T ref : registry) {
  			if(areEqual(instance, ref)) {
  				return true;
  			}
  		}
  		return false;
  	}
docs: Checks whether the given instance is already
  contained in the registry.
  
  @param instance Instance to check against registry.
  @return Whether the given instance is already registered.