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