A structure that will be passed to invocation of
MeasuredProcess.next. Not intended for inspection externally, contains
implementation details of the process.
The result of the process given the current input and state. Using
the rules of composition, either passed to input arguments of chained a
MeasuredProcess, or concatenated with outputs of parallel
Metrics derived from the computation of result. Intended for
surfacing values to track the progress of a process that are not sent to