Look up function:
classThis engine transitions from its initial state['a]epsilon_engine :'a engine_state -> Unixqueue.event_system ->['a]engine
`Working 0 in one
step to the passed constant state.
In previous versions of this library the class was called const_engine.
However, this is not a constant thing. In particular, it is possible
that this engine is aborted, so the passed state is not reached.
To avoid programming errors because of the misnomer, this class has been
renamed.
