public default <T extends TraversalSource> T begin() {
        return (T) begin(GraphTraversalSource.class);
    }
Starts a transaction in the context of a {@link GraphTraversalSource} instance. It is up to the
{@link Transaction} implementation to decide what this means and up to users to be aware of that meaning.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/Transaction.java