For a comprehension `[x for x in l]
- `l` is now a legal argument (in DataFlowPublic)
- `l` is the argument of the comprehension function (in DataFlowDispatch)
- the parameter of the comprehension function is being read rather than `l` (in IterableUnpacking)
Thus the read that used to cross callable boundaries is now split into a arg-param edge and a read from that param.