Step - index

index() maps a collection to the list of its [element, position] pairs, positions counted from 0 (TinkerPop IndexStep with its default list indexer).

g.V().hasLabel("software").values("name").fold().index().to_list()   // [[["lop", 0], ["ripple", 1]]]
g.V("1").values("age").index().to_list()                             // [[[29, 0]]]
g.V("1").valueMap("name", "age").index().to_list()
// [[[{"name": ["marko"]}, 0], [{"age": [29]}, 1]]]

• A list and a path are indexed element by element.
• A map is indexed entry by entry; each entry is a one-entry map.
• Any other value (a number, a string, a vertex, null) is a one-element collection, so g.V().hasLabel("software").index().unfold() yields [v[3], 0] and [v[5], 0].

with(WithOptions.indexer, WithOptions.map) (see with(WithOptions..)) builds a {position: element} map instead:

g.V().hasLabel("person").values("name").fold().order(Scope.local).
  index().with(WithOptions.indexer, WithOptions.map).next()
// {0: "josh", 1: "marko", 2: "peter", 3: "vadas"}

Rust: GraphTraversalSource::index(), __::index(), followed by with_option(WithOptions::Indexer, Some(WithOptions::Map)).