protected Map<?,?> resolveVertices(final Map map, final Traverser.Admin<S> traverser) {
        resolveVertex(Merge.outV, Direction.OUT, map, traverser, outVTraversal);
        resolveVertex(Merge.inV, Direction.IN, map, traverser, inVTraversal);
        return map;
    }
Translate the Map into search criteria. Default implementation is to translate the Map into a g.E() or
g.V().out/inE() traversal. Graph providers will presumably optimize this traversal to use whatever indices are
present and appropriate for efficiency.

Callers are responsible for closing this iterator when finished.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MergeEdgeStep.java