Doctest: Expression.is_positive/negative fixed

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The two functions that query Pynac expressions' info::flags only work with numerics and symbols with domain. The rest simply returns False:

sage: (1-pi).is_negative()
False
sage: (log(1/2)).is_negative()
False
sage: e.is_positive()
False
sage: (e+1).is_positive()
False
sage: (2*e).is_positive()
False
sage: (e^3).is_positive()
False

UPDATE: everything above but the first works now.

Depends on #24497

CC: @kcrisman @pelegm

Component: symbolics

Author: Ralf Stephan

Branch/Commit: 8e446e6

Reviewer: Peleg Michaeli, Vincent Delecroix

Issue created by migration from https://trac.sagemath.org/ticket/18630

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Changed upstream from Not yet reported upstream; Will do shortly. to Fixed upstream, but not in a stable release.

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comment:1

Parts of this are fixed in pynac/pynac#66 (will be pynac-0.3.9.1/0.4.1)

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Dependencies: pynac-0.3.9.1

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comment:2

While products are okay already, sums with negative terms are not and will not be for some time. Correct handling depends on pynac/pynac#67 and pynac/pynac#68 and affects #12588 too, for example.

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Description changed:

--- 
+++ 
@@ -15,4 +15,4 @@
 False
 ```
 
-
+UPDATE: everything above but the first works now.

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Changed upstream from Fixed upstream, but not in a stable release. to Reported upstream. Developers acknowledge bug.

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Changed dependencies from pynac-0.3.9.1 to none

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comment:4

Since everything apart from sums works now, let's look at what is possible with sums containing exact numerics (in the Pynac sense) and constants only. The border case is if the expression is nonzero---but only at certain precision. This is impossible to handle. It is also not possible to convert to float and only check sign if the absolute result is greater some epsilon, because that could be due to precision loss in a denominator.

I'm therefore just adding the doctests here to resolve the ticket.

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Changed upstream from Reported upstream. Developers acknowledge bug. to none