Availability:
:- use_module(library(http/http_session)).
[semidet]http_in_session(-SessionId)session(ID) from the
current HTTP request (see http_current_request/1).
The value is cached in a backtrackable global variable http_session_id.
Using a backtrackable global variable is safe because continuous worker
threads use a failure driven loop and spawned threads start without any
global variables. This variable can be set from the commandline to fake
running a goal from the commandline in the context of a session.