This is a PEBKAC, not a problem with REPL-driven development. He had all the test bodies written as input in the REPL, he had all the test results printed as ou

This is a PEBKAC, not a problem with REPL-driven development. He had all the test bodies written as input in the REPL, he had all the test results printed as output in the REPL. Why didn't he turn these into unit tests as he programmed? That's a simple lack of foresight on the programmer's side that is then blamed on the tools that were used.