public Builder productiveKeys(final String key, final String... rest) {
            this.productiveKeys.clear();
            productiveKeys.add(key);
            productiveKeys.addAll(Arrays.asList(rest));
            return this;
        }
Specify the list of property keys that should always be productive for {@code by(String)}. If the keys are
not set, then all `by(String)` are productive. Arguments to {@code by()} that are not of type
{@link ValueTraversal} will not be considered.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/strategy/optimization/ProductiveByStrategy.java