downlinks

In std/graph.

fn downlinks(x: Option<Doc>) : List<Edge>
fn downlinks(x: Doc) : List<Edge>
fn downlinks(x: Hit<Card>) : List<Edge>
fn downlinks(xs: Collection<Doc>) : List<Edge>
fn downlinks(xs: Collection<Hit<Card>>) : List<Edge>

The edges of the links other cards of the vault write to x: one Edge per authored link, its target the card x. Over a hit, the card it holds; over an absent card, none; over a collection, every element's, in order. It needs a vault: a document alone has nothing that points at it.