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Function nan/0
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LogicalCaptain said (2020-03-26T08:26:27):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain. 
?- NAN is nan, write_canonical(NAN).
1.5NaN
NAN = 1.5NaN.

Weird!

I suppose one can test against NaN using =:= ?