It is an error if one of the mprocs does not accept as arguments the number of values wrapped by Just/Right produced by its predecessor.
Monadic bind. If maybe/either is Nothing/ a Left, it is returned as is. If maybe/either is a Just/Right, it behaves as if they invoked maybe-compose/either-compose on mprocs and applied the resulting mproc to the payload of the Just/Right, returning the result Maybe/Either.