code: protected void mergeImmutableContainerTypesFrom(Configuration otherConfiguration) {
          Set<Dotted> union =
                  Sets.union(hardcodedImmutableContainerClasses.build(), otherConfiguration.immutableContainerClasses());
  
          hardcodedImmutableContainerClasses = ImmutableSet.<Dotted>builder().addAll(union);
      }
docs: Merges the immutable container types of this Configuration with the given
  Configuration.
  
  The resultant immutable container types results will be the union of the two sets of
  immutable container types. Where the type is found in both
  Configurations, the result from otherConfiguration will replace the
  existing result in this Configuration. This replacement behaviour will
  occur for subsequent calls to
  {@link #mergeImmutableContainerTypesFrom(Configuration)} .
  
  @param otherConfiguration - Configuration to merge immutable container types with.