@Override
            public K getKey() {
              return backingEntry.getKey();
            }

            @Override
            public ImmutableSet<V> getValue() {
              return ImmutableSet.of(backingEntry.getValue());
            }
          };
        }
      };
    }

    // redeclare to help optimizers with b/310253115
    @SuppressWarnings("RedundantOverride")