Stubs

Each query has been defined in form of a predicate, with input and output parameters. Regardless of the actual function of the query, it is easy to generate a predicate which syntactically behaves like the finished program. It reads the input parameters and creates output terms which satisfy the specification. Initially, it does not matter if the output parameters are not consistent with the input values, as long as their form is correct.

If a top-level query has already been revised into smaller components, we can immediately write the body of the top-level predicate calling the individual components in the right order and with the correct parameters. Adding stub definitions of these components again leads to an executable program.

Whenever we finish development of some of the components, we can immediately replace the stub code with the working implementation. Provided that the inputs are sufficiently simple, we get a simulated version of our application that we can convert piece by piece into the real application.