Opaque OrdinalPool.getThreadR.

Lemma updThread_internal_step:
forall tp tp' m m' i j c pmap
(Hneq: i <> j)
(cnti: containsThread tp i)
(cnti': containsThread tp' i)
(cntj: containsThread tp j)
(cntj': containsThread (updThread cnti c pmap) j)
(Hcomp: mem_compatible tp m)
(Hcomp': mem_compatible (updThread cnti c pmap) m)
(Hinv': invariant (updThread cnti c pmap))
(Hstep: internal_step cntj Hcomp tp' m'),
internal_step cntj' Hcomp' (updThread cnti' c pmap) m'.