public <S, E> List<E> get(final Traverser.Admin<S> traverser, final Object key, final Supplier<E> defaultValue) {
        final List<E> values = (List<E>) this.parameters.get(key);
        if (null == values) return Collections.singletonList(defaultValue.get());
        final List<E> result = new ArrayList<>();
        for (final Object value : values) {
            result.add(value instanceof Traversal.Admin ? TraversalUtil.apply(traverser, (Traversal.Admin<S, E>) value) : (E) value);
        }
        return result;
    }
Gets the list of values for a key, while resolving the values of any parameters that are {@link Traversal}
objects.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/Parameters.java