[Issue 13936] groupBy must be redone

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=13936

--- Comment #15 from hsteoh at quickfur.ath.cx ---
I'm OK with breaking up the current somewhat messy code to make a separate
groupByAdjacent for the non-equivalence relation implementation. In fact, I had
actually implemented that, but reverted it because people objected. *shrug*

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