NA & operations with it

[Daniel Kraft]

Hi!

I just chased a bug in my Octave codes, and found that it was related to
my usage of NA to denote special cases for entries of some data
structures.  In particular, I found that -NA = NaN and not NA again.

So far, so good.  However, I experimented a bit further, and found that
"almost all" other (linear) operations with NA in fact return NA again:

+NA, 0 - NA, 1 * NA, Inf + NA, NaN * NA, and others

So why is just -NA not NA but NaN instead?  Is this a deliberate choice,
or just an edge case no-one has thought about so far?  I take it that
one should probably not rely on any of those (and I changed my code now
accordingly), but it still seems curious to me; and if there was no
actual reason for this behaviour, I would suggest to change it to make
it consistent.  (I can try to provide a patch.)

Yours,
Daniel

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