Design:
- Add one to each item in a list.
- Add one to each item in a list, in place.
for ii in range(len(xs)):
Distraction:
- Let’s make a grid of colored cells.
Concepts:
- Mutation
- Mutability
- List are, tuples aren’t.
- Pass by value vs. reference
- Unexpected mutation
- Nested mutation
- Reassignment
