There are some steps that empty out the path, e.g. `mean`:
gremlin> g.E().as("a").values("weight").as("b").path()
==>[e[7][1-knows->2], 0.5]
==>[e[8][1-knows->4], 1.0]
==>[e[9][1-created->3], 0.4]
==>[e[10][4-created->5], 1.0]
==>[e[11][4-created->3], 0.4]
==>[e[12][6-created->3], 0.2]
gremlin> g.E().as("a").values("weight").mean().as("b").path()
==>[0.5833333333333334]
Question: where does this 'emptying out' happen? How can I tell which
steps have this behaviour?
Background: Gremlin-Scala preserves the type of every labelled step, so
that the `select` step returns a typed list (HList) of the labelled types.
Cheers
Michael
