This lecture, we talked about the idea of work, which is the run-time
cost of a function, defined in terms of abstract units. We computed it via
solving *recurrences* we obtained from the definition of SML functions,
and obtained Big-O bounds, as a result.

We then moved on to the idea of *parallel computing*, and saw that
this idea of solving recurrences generalized to *span*, which is the
cost of running a computation with access to infinitely many parallel processors.

The central thesis: using mathematical techniques, we can formally prove the runtime of our functions!