Once the primitives of the DP have been defined it can be used just as an ordinary function:
This would return true if there are marsh and forest regions which overlap. Such a function call could occur wherever an ordinary Boolean would normally be found --- for instance in the test part of a conditional statement.
(The example illustrates that it may be possible to provide a facility which would enable one to evaluate a formula with respect to more than one theory --- in this case topology and land_types. This would only be possible if the two theories are logically independent in that the primitive concepts of one theory have no connection with those of the other.)