middle (MidAssign lhs expr) = gen expr . kill lhs
middle (MidStore addr rval) = gen addr . gen rval
middle (MidUnsafeCall tgt ress args) = gen tgt . gen args . kill ress
+ middle (MidAddToContext ra args) = gen ra . gen args
middle (CopyIn _ formals _) = kill formals
middle (CopyOut _ actuals) = gen actuals