[m-rev.] for review: remove "is" as synonym for "="

[Zoltan Somogyi]

For review by anyone. The second part will be committed
after the first is in an rotd.

BTW, I couldn't find any actual documentation of "is"
as a synonym for "=". It is not among the goal kinds in
section 2.11 of the reference manual, and a search for "{is}"
did not find anything relevant. (A search for plain "is" would
yield far too many false hits to be useful.)

Zoltan.
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