analyzing every aspect of a complex system, a practical approach is to use different methods in combination. When
combining methods it is important to
consider both
—finding a suitable style for using different methods together; and
—finding a suitable meaning for using
different methods together.
Very often neither is addressed adequately. Failure to find a suitable style
misses out on the true advantages of
combining methods. For example, the Z
school stresses the importance of presentation of specifications in an accessible form, with plenty of natural language. This emphasis has helped in
popularizing its notation. Any combination must preserve this style of presentation.
Failure to attend to the theoretical
foundations of the combination misses
out on the true advantages of formality.
In chemistry, a distinction is drawn between a mixture and a compound. In a
mixture, the ingredients merely mingle
together; in a compound, the ingredients become chemically united. So it is
with combining different formal methods. If the meaning of the combination
is not properly explained, then the result is merely a mixture: nothing more
can be deduced from the joint description than from the separate ones. If the
meaning of the combination is explained, then the result is much more
powerful. It then becomes possible to
have two views of a system specification, and to reason with and refine one
view, and to understand the consequences in the other view