
constraint is tested.
It is hard to define exactly what the difference is between this kind of constraint
and constraints explicitly in the design. It is clear that the data-base constraints will
often be about more global attributes and their limits, and will be tests that one would
not normally include explicitly in one's design procedures. However, if a piece of
constraining knowledge is activated in this jack-in-the-box fashion often enough, the
designer will tend to remember it and include it in the design activity. This is
especially true, of course, if it has often failed. Consequently, as a result of
experience, constraints can "migrate ...