@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 Vertex#addEdge(String, Vertex, Object...)}. 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 edge is unique to appropriately apply
assertions. Graph providers, especially those who support schema features, may have other methods for
uniquely identifying a edge 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