Part of the initial progress in developing an engineering basis for software development was to identify the specific tasks at hand and highlight their differences. Although that step was probably inevitable, it has led to a somewhat skewed view of software engineering, which ignores the fundamental unity of software construction, and leads to unnecessary gaps, detrimental to quality and productivity. It is more fruitful to take advantage of the fundamental invariants of software development and view system engineering as a continuous, seamless and reversible process. The talk will show how that full roundtrip engineering is possible in practice, leading to far higher quality of both process and product.