doctest fix for: substitute exp(-x) with x infinity fails

[videlec]

sage: exp(-Infinity)
0

is fine, but

sage: exp(-x).subs(x=Infinity)
Traceback (most recent call last):
...
RuntimeError: exp_eval(): exp^(unsigned_infinity) encountered 

Original report thread:

https://groups.google.com/forum/#!topic/sage-devel/tqDSbZ499ME

Depends on #19948

CC: @rwst

Component: symbolics

Keywords: bug

Author: Benjamin Hackl

Branch/Commit: 45420f5

Reviewer: Ralf Stephan

Issue created by migration from https://trac.sagemath.org/ticket/19918

[videlec]

Description changed:

--- 
+++ 
@@ -11,3 +11,7 @@
 ...
 RuntimeError: exp_eval(): exp^(unsigned_infinity) encountered 
 ```
+
+Original report thread:
+
+    https://groups.google.com/forum/#!topic/sage-devel/tqDSbZ499ME

[jdemeyer]

comment:2

This is very likely a bug in pynac.

[behackl]

comment:3

As far as I can tell, there is some sort of sign confusion occurring in pynac. This is now pynac/pynac#123.

[behackl]

Upstream: Reported upstream. Developers acknowledge bug.

[rwst]

comment:4

The reason is

sage: hash(SR(-oo))
2886518159
sage: hash((-x).subs(x=oo))
81231252
sage: hash((x).subs(x=-oo))
2886518159

See #19927 and #19928 for related bugs.

[behackl]

Dependencies: pynac-0.6.0

[behackl]

Changed upstream from Reported upstream. Developers acknowledge bug. to Fixed upstream, in a later stable release.

[behackl]

comment:5

This will be fixed in pynac-0.6.0; I'll add a doctest on this ticket after the upgrade.

[behackl]

Changed dependencies from pynac-0.6.0 to pynac-0.6.1

[behackl]

comment:6

Replying to @behackl:

This will be fixed in pynac-0.6.0; I'll add a doctest on this ticket after the upgrade.

... typo; I meant the next release (pynac-0.6.1 or so...).

[behackl]

New commits:

5c88141	19448: update Pynac pkg data
b76a8ad	19948: add stieltjes() interface; fix doctest
4857402	19949: fix type of error raised by stieltjes()
be7423b	add doctest for exp (substitution with oo)
f2546c0	add doctest for arctan (substitution with oo)