What this allows us to do is much better relationship tracking between
sessions and invoices. The CASS file already has all the structure we
need, and it would be silly to replicate it via complicated time
association algorithms. This approach uses the linear nature of the
data file to track the position relative to other entries.
The timeline module is then responsible for making changes to the
internal representation (in case it is being used as a library for
multi-query commands), and emitting a `Delta` type that can be used to
easily patch the IR in question, because the mapping between the
timeline and IR representations is linear.
Katharina Fey