enum ValueConsumer {
        DisallowUnsafeRead, IgnoreUnsafeRead
    }

    /**
     * Carries information about the producer of a value.
     */
    interface ValueProducer {
        NoProducer NO_PRODUCER = new NoProducer();
        UnknownProducer UNKNOWN_PRODUCER = new UnknownProducer();

        default boolean isKnown() {
            return true;
        }

            return ExecutionTimeValue.fixedValue(emptyCollection());
        }

        @Override
        public ValueProducer getProducer() {
            return ValueProducer.noProducer();
        }

        @Override
        public String toString() {
            return "[]";
        }
    }

            return ExecutionTimeValue.fixedValue(ImmutableMap.of());
        }

        @Override
        public ValueProducer getProducer() {
            return ValueProducer.noProducer();
        }

        @Override
        public String toString() {
            return "{}";
        }
    }

    private static class FixedSupplier<K, V> implements MapSupplier<K, V> {