As-if!
'… we need to accept the possibility that we will iterate during the development, perhaps within a given phase (eg reworking a design until it seems that it will give the required performance), or from one phase back to another (eg to correct a faulty design decision that, during construction, is exposed by a failure to be able to implement it). The biggest iteration is the one where we complete the development and, when we see what we have produced, we decide we need to start again and redefine the user requirements in the light of what we have produced. Such iterations happen in the real world and we wanted to accept this fact and put it in our model. Royce first formulated this idea in what he labelled the waterfall model … page 31
'The "oldest" process model is the waterfall model … This shows a similar progression from inception, through definition, the various stages of design, code, unit testing and integration to final acceptance. The fact that that progression is never smooth and monotonic is acknowledged by drawing little backward arrows suggesting iteration." page 39
1990 Strategies for Software Engineering: The Management of Risk and Quality
https://www.google.com/books/edition/Strategies_for_Software...