��A coding experiment.
Consider a Scrum development environment. Every programming team has an end user as a member.��
The team's task is to code a credit card validity check.
A first goal is that the user representative shall read the code and agree that it is a correct rendering of their code checker:

�� luhnTest: trialNumber
�� | s1 odd s2 even charValue reverse |
-----------------------------------------------
" Luhn test according to Rosetta"
"Reverse the order of the digits in the number."
�� reverse := trialNumber reversed.
"Take the first, third, ... and every other odd digit in the reversed digits and sum them to form the partial sum s1"
�� s1 := 0.
�� odd := true.
�� reverse do:
�� [:char |
�� odd
�� ifTrue: [
�� s1 := s1 + char digitValue.
�� ].
�� odd := odd not
�� ].
"Taking the second, fourth ... and every other even digit in the reversed digits:
Multiply each digit by two and sum the digits if the answer is greater than nine to form partial sums for the even digits"
�� "The subtracting 9 gives the same answer. "
"Sum the partial sums of the even digits to form s2"
�� s2 := 0.
�� even := false.
�� reverse do:
�� [:char |
�� even
�� ifTrue: [
�� charValue := char digitValue * 2.
�� charValue > 9 ifTrue: [charValue := charValue - 9].
�� s2 := s2 + charValue
�� ].
�� even := even not
�� ].
"If s1 + s2 ends in zero then the original number is in the form of a valid credit card number as verified by the Luhn test."
�� ^(s1 + s2) asString last = $0
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Once this step is completed, the next step will be to make the code right without altering the algorithm (refactoring). The result should be readable and follow the team's conventions.
��

P.S. code attached.