@FeatureDescriptor(name = FEATURE_UPSERT)
            default boolean supportsUpsert() {
                return false;
            }
Determines if the {@code Graph} implementation uses upsert functionality as opposed to insert
functionality for {@link #addVertex(String)}. This feature gives graph providers some flexibility as
to how graph mutations are treated. For graph providers, testing of this feature (as far as TinkerPop
is concerned) only covers graphs that can support user supplied identifiers as there is no other way
for TinkerPop to know what aspect of a vertex is unique to appropriately apply assertions. Graph
providers, especially those who support schema features, may have other methods for uniquely identifying
a vertex and should therefore resort to their own body of tests to validate this feature.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/Graph.java