@Override
    public void apply(final Traversal.Admin<?, ?> traversal) {
        if (!traversal.getStrategies().getStrategy(ComputerFinalizationStrategy.class).isPresent() &&
                !traversal.getStrategies().getStrategy(ComputerVerificationStrategy.class).isPresent()) {
            if (!TraversalHelper.getStepsOfAssignableClass(VertexComputing.class, traversal).isEmpty())
                throw new VerificationException("VertexComputing steps must be executed with a GraphComputer: " + TraversalHelper.getStepsOfAssignableClass(VertexComputing.class, traversal), traversal);
        }

        for (final Step<?, ?> step : traversal.getSteps()) {
            for (String label : new HashSet<>(step.getLabels())) {
                if (Graph.Hidden.isHidden(label))
                    step.removeLabel(label);
            }
            if (step instanceof ReducingBarrierStep && step.getTraversal().getParent() instanceof RepeatStep && step.getTraversal().getParent().getGlobalChildren().get(0).getSteps().contains(step))
                throw new VerificationException("The parent of a reducing barrier can not be repeat()-step: " + step, traversal);

            // prevents silly stuff like g.V().emit()
            if (step instanceof RepeatStep && null == ((RepeatStep) step).getRepeatTraversal())
                throw new VerificationException("The repeat()-traversal was not defined:" + traversal, traversal);

        }

        // The ProfileSideEffectStep must be one of the following
        // (1) the last step
        // (2) 2nd last step when accompanied by the cap step or none step (i.e. iterate() to nothing)
        // (3) 3rd to last when the traversal ends with a RequirementsStep.
        final Step<?, ?> endStep = traversal.asAdmin().getEndStep();
        if (TraversalHelper.hasStepOfClass(ProfileSideEffectStep.class, traversal) &&
                !(endStep instanceof ProfileSideEffectStep ||
                        (endStep instanceof SideEffectCapStep && endStep.getPreviousStep() instanceof ProfileSideEffectStep) ||
                        (endStep instanceof DiscardStep && endStep.getPreviousStep() instanceof SideEffectCapStep && endStep.getPreviousStep().getPreviousStep() instanceof ProfileSideEffectStep) ||
                        (endStep instanceof RequirementsStep && endStep.getPreviousStep() instanceof SideEffectCapStep && endStep.getPreviousStep().getPreviousStep() instanceof ProfileSideEffectStep) ||
                        (endStep instanceof RequirementsStep && endStep.getPreviousStep() instanceof DiscardStep && endStep.getPreviousStep().getPreviousStep() instanceof SideEffectCapStep && endStep.getPreviousStep().getPreviousStep().getPreviousStep() instanceof ProfileSideEffectStep))) {
            throw new VerificationException("When specified, the profile()-Step must be the last step or followed only by the cap()-step.", traversal);
        }

        if (TraversalHelper.getStepsOfClass(ProfileSideEffectStep.class, traversal).size() > 1) {
            throw new VerificationException("The profile()-Step cannot be specified multiple times.", traversal);
        }
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Stephen Mallette (http://stephen.genoprime.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/strategy/verification/StandardVerificationStrategy.java