  To begin with, it is pointed out that the form of the quantum probabil- ity
formula originates in the very initial state of the object system as seen when
the state is expanded with the eigen-projectors of the measured ob- servable.
Making use of the probability reproducibility condition, which is a key concept
in unitary measurement theory, one obtains the relevant coher- ent distribution
of the complete-measurement results in the final unitary- measurement state in
agreement with the mentioned probability formula. Treating the transition from
the final unitary, or premeasurement, state, where all possible results are
present, to one complete-measurement result sketchily in the usual way, the
well-known probability formula is derived. In conclusion it is pointed out that
the entire argument is only formal unless one makes it physical assuming that
the quantum probability law is valid in the extreme case of probability-one
(certain) events (projectors).
