https://www.google.com/search?q=images+of+stages+of+program+development&tbm=isch&source=univ&sa=X&ved=2ahUKEwjRo56Jh-3gAhX2BGMBHSn7CYQQ7Al6BAgEEA8&biw=1366&bih=657
Longhorn Computer studies, page 50-52
Log On computer studies, page 45-48
Elementary Programming Principles
References & further reading
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