# Objective
- First step towards #279
## Solution
Makes the necessary internal data structure changes in order to allow
system removal to be added in a future PR: `Vec`s storing systems and
system sets in `ScheduleGraph` have been replaced with `SlotMap`s.
See the included migration guide for the required changes.
## Testing
Internal changes only and no new features *should* mean no new tests are
requried.
