union() as a Start Step

g.union(branches...) starts a traversal without V(). It differs from Apache TinkerPop 3.8.2 in two details.

The seed is null

• TinkerPop: the start union has no incoming traverser of its own.
• Graphersal: a root traversal whose first step is union() gets exactly one seed traverser holding null; child traversals are never seeded. A branch such as __.identity() therefore yields that null: g.union(__.identity()) returns one null.

A branch that opens with inject() is its own start

• Graphersal: such a branch runs over an empty source instead of the seed.
• Why: otherwise g.union(__.inject(1), __.inject(2)) would echo the seed and return null, 1, null, 2 instead of 1, 2. In the middle of a traversal inject() still emits the incoming traverser as well.

g.union(__.inject(1), __.inject(2)).toList()   // 1, 2
g.union(__.identity()).toList()                // null
g.union(__.V("1"), __.V("4")).values("name").toList()

Nested V()/E() in a branch start from the incoming traverser and extend its path.