Opaque containsThread.

Lemma step_schedule:
forall tpc tpc' mc mc' i U1 U2 U1' tr tr'
(Hstep: MachStep (i :: U1, tr, tpc) mc (U2, tr ++ tr', tpc') mc'),
exists U2',
MachStep (i :: U1', tr, tpc) mc (U2', tr ++ tr', tpc') mc'.