It’s called a body less clause, and in this case used only to name params for the docs, as they are used only as patterns in the following clauses and the names given by the algorithm were not very good.

This kind of vodyless clause is optional.

If though you do use default arguments in functions that have multiple clauses a bodyless clause specifying the default arguments is mandatory.

You can only have one default clause per function, so if you have many heads then you put it on the bodyless clause.

Thanks! Now that I know the name, “bodyless clause”, I
found something about this bugger.

None of my Elixir books or searches of “def without do”
or similar guesses offered any illumination.

Thanks again!