A snapshot and a project, bound once. Of the wire's methods, most take `snapshot` and `project` as their first two arguments. They say which program is being asked, not what is being asked of it, and they do not vary across the calls a caller makes against one program - so `Session` holds them and the members below take only what is left. It is the same move `Node<'Tag>` makes for a source file and an index. The type parameter is the transport. `Session` answers synchronously and `Session` in `Async`; the member names and argument lists are identical, so the two differ only in what they return. Build one with `channel.Session(...)` or `mailbox.Session(...)`. Handles mean something only within the program that produced them, so a session is also the scope in which a symbol or type id is valid. A response that names a different project - `SymbolResponse.Project`, which the schema documents as the default for follow-up lookups - is followed with `ForSymbol`, not by passing a project argument.