Session::Controllables is a shared_ptr<> list. As long
as the session exists the Controllables will be around. Destroyed(*)
can only be called after the session is destroyed and releases the
shared_ptr<>
NB. this code had a nice hack to construct a "shared_from_this"
workaround. For future reference:
struct null_deleter { void operator()(void const *) const {} };
boost::shared_ptr<Controllable>(c, null_deleter())
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