private Optional<String> lookup(final ReferenceData referenceData,
                                    final List<PipelineReference> pipelineReferences,
                                    final String time,
                                    final String mapName,
                                    final String key) {
        LOGGER.debug("Looking up {}, {}, {}", time, mapName, key);
        Optional<String> optValue = lookup(referenceData,
                pipelineReferences,
                DateUtil.parseNormalDateTimeString(time),
                mapName,
                key);
        LOGGER.debug("Found {}", optValue.orElse("EMPTY"));
        return optValue;
    }
Make sure that it copes with a map not being in an early stream but is in a later one.
stroom-app/src/test/java/stroom/pipeline/refdata/TestReferenceData.java