Description:

Often in program logic, multiple different steps need to be taken that
are independent of each other, but their total result is needed before
the next step can be taken. In synchonous code, the usual approach is
to do them sequentially.

An asynchronous or event-based program could do this, but if each step
involves some IO idle time, better overall performance can often be
gained by running the steps in parallel. A Async::MergePoint object
can then be used to wait for all of the steps to complete, before
passing the combined result of each step on to the next stage.