Augment, Never Replace, Natural Language says to create requirements specifications that contain both natural language and formal models. Let's go a step further: Always create the natural language version of a requirements specification first. If you write the formal model first, the tendency will be to write the natural language that describes the model instead of the desired system. This also applies to writing test cases. Write the test cases first to avoid the tendency to only test the code that was written or the way it is applied.
The best approach is to (1) write the natural language, (2) write the formal model, and (3) adapt the natural language to reduce ambiguities that become apparent when writing the formal model.
