This question already has an answer here:
This question I asked here:
Has not been well received (-2, and two close votes for requesting tutorials/tools, at time of posting this meta question).
The question asks for a canonical reference/answer for 'What is a reducer function'?
A comment given points us to the term 'Fold' and a wikipedia article for it - which precisely gets to what I was asking.
Is the question I asked off topic?