Remark perm_order_dec:
  forall p1 p2, {perm_order p1 p2} + {~perm_order p1 p2}.Remark perm_order'_dec:
  forall op p, {perm_order' op p} + {~perm_order' op p}.

Theorem perm_dec: forall m b ofs k p, {perm m b ofs k p} + {~ perm m b ofs k p}.