pub const AT: &str = "at";Expand description
(at "ana" "/done") — a session slot that names a route as well as an actor.
The slot is one node with two shapes rather than two slots, because every form that has one
already treats it as optional: a bare string is session("ana") and this is
session("ana", "/done"). A second positional slot would have made “no actor, a route” and
“an actor, no route” the same arity and told them apart by which one happened to be a
string.