public void setEmitTraversal(final Traversal.Admin<S, ?> emitTraversal) {
        if (null != this.emitTraversal)
            throw new IllegalStateException("The repeat()-step already has its emit()-modulator declared: " + this);
        if (null == this.repeatTraversal) this.emitFirst = true;
        this.emitTraversal = this.integrateChild(emitTraversal);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/branch/RepeatStep.java