@Test
    @FeatureRequirement(featureClass = Graph.Features.GraphFeatures.class, feature = FEATURE_TRANSACTIONS)
    public void shouldHaveExceptionConsistencyWhenTransactionAlreadyOpen() {
        if (!g.tx().isOpen())
            g.tx().open();

        try {
            g.tx().open();
            fail("An exception should be thrown when a transaction is opened twice");
        } catch (Exception ex) {
            validateException(Transaction.Exceptions.transactionAlreadyOpen(), ex);
        }
    }
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-test/src/main/java/org/apache/tinkerpop/gremlin/structure/TransactionTest.java