We can make ``inf - inf'` be any real number we like, say,
a, just by claiming that we added a to the first
infinity but not to the second. This is just as true for complex
values of a, so `nan`

can stand for a complex number.
(And, similarly, `uinf`

can stand for an infinity that points
in any direction in the complex plane, such as ``(0, 1) inf'`).

In fact, we can multiply the first `inf`

by two. Surely
``2 inf - inf = inf'`, but also ``2 inf - inf = inf - inf = nan'`.
So `nan`

can even stand for infinity. Obviously it's just
as easy to make it stand for minus infinity as for plus infinity.

The moral of this story is that "infinity" is a slippery fish
indeed, and Calc tries to handle it by having a very simple model
for infinities (only the direction counts, not the "size"); but
Calc is careful to write `nan`

any time this simple model is
unable to tell what the true answer is.

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