@Override
    public void addChildOption(final Merge token, final Traversal.Admin<S, C> traversalOption) {
        if (token == Merge.onCreate) {
            this.onCreateTraversal = this.integrateChild(traversalOption);
        } else if (token == Merge.onMatch) {
            this.onMatchTraversal = this.integrateChild(traversalOption);
        } else {
            throw new UnsupportedOperationException(String.format("Option %s for Merge is not supported", token.name()));
        }
    }
Determine if this is the first pass through {@link #processNextStart()}.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MergeElementStep.java