I had to do some more research on this. I suddenly didn't understand why the
query in question:
```java
g.V().
project('v','edges').
by(valueMap()).
by(outE().valueMap().fold())
```
from https://stackoverflow.com/q/55571767/1831717
produces a `Set` for `SetIO` to be the source of this problem. So I tested the
traversal in the Console:
```text
gremlin> x = g.V().store('vertices').by(valueMap()).
......1> outE().store('edges').by(valueMap()).
......2> cap('vertices','edges').next()
==>vertices={{name=[marko], age=[29]}=1, {name=[vadas], age=[27]}=1,
{name=[lop], lang=[java]}=1, {name=[josh], age=[32]}=1, {name=[ripple],
lang=[java]}=1, {name=[peter], age=[35]}=1}
==>edges={{weight=0.4}=2, {weight=0.5}=1, {weight=1.0}=2, {weight=0.2}=1}
gremlin> x.getClass()
==>class java.util.HashMap
gremlin> x.vertices.class
==>class org.apache.tinkerpop.gremlin.process.traversal.step.util.BulkSet
gremlin> x.vertices[0].getClass()
==>class java.util.HashMap
gremlin> x.vertices[0].age.class
==>class java.util.ArrayList
```
The only collection that is `Set` based is "vertices" and "edges" keys which
are `BulkSet`. The 3.3.x line of code does not have support for this `BulkSet`
type - this was rectified in 3.4.0:
https://github.com/apache/tinkerpop/blob/3.4.1/CHANGELOG.asciidoc#tinkerpop-340-release-date-january-2-2019
So, technically the issue is resolved by 3.4.0+ but would require that you and
Neptune both use this newer version. I did come up with a workaround if to
transform the `BulkSet` with Gremlin:
```text
gremlin> x = g.V().store('vertices').by(valueMap()).
......1> outE().store('edges').by(valueMap()).
......2> cap('vertices','edges').
......3> select('vertices','edges').
......4> by(unfold().fold()).toList()
==>[vertices:[[name:[marko],age:[29]],[name:[vadas],age:[27]],[name:[lop],lang:[java]],[name:[josh],age:[32]],[name:[ripple],lang:[java]],[name:[peter],age:[35]]],edges:[[weight:0.4],[weight:0.4],[weight:0.5],[weight:1.0],[weight:1.0],[weight:0.2]]]
gremlin> x.vertices.class
==>class java.util.ArrayList
```
I would think that this would work on 3.3.x without any changes.
[ Full content available at: https://github.com/apache/tinkerpop/pull/1095 ]
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