Remark in_srcs:
  forall mu s, In s (srcs mu) -> exists d, In (s, d) mu.Remark in_dests:
  forall mu d, In d (dests mu) -> exists s, In (s, d) mu.

Lemma parmove_srcs_initial_reg_or_temp: forall mu s, In s (srcs (parmove mu)) -> In s (srcs mu) \/ is_temp s.