Wording in sections 2 and 3, with attention on a couple of points:
Wording in sections 2 and 3, with attention on a couple of points:

  - don't even suggest that the empty fact is an appropriate
    start fact for constant propagation

  - the example that mentioned loops wasn't a loop

  - point out in section 3.1 that shape is a new thing

hvr/hoopl,ezyang/hoopl,hvr/hoopl