No operation required by
copyable<remove_cvref_t<Sch>> and
equality_comparable<remove_cvref_t<Sch>>
shall exit via an exception.
None of these operations,
nor a scheduler type's schedule function,
shall introduce data races
as a result of potentially concurrent ([intro.races]) invocations
of those operations from different threads.
For any two values sch1 and sch2
of some scheduler type Sch,
sch1 == sch2 shall return true
only if both sch1 and sch2 share
the same associated execution resource.
For a given scheduler expression sch,
if the expression get_domain(sch) is well-formed,
then the expression get_domain(get_env(schedule(sch)))
is also well-formed and has the same type.