doctest fix for: symbolic matrices fail operations minus and mul

[rwst]

In examples like this:

sage: f(x)=matrix()
sage: bool(f(x)-f(x)==0)
...
TypeError: mutable matrices are unhashable
sage: f(x)*1
...
TypeError: mutable matrices are unhashable

a Pynac bug manifests, see pynac/pynac#34

This ticket should provide doctests, it depends on pynac-0.4.2/0.3.9.2.

Depends on #17321

CC: @nbruin

Component: symbolics

Author: Ralf Stephan

Branch/Commit: 919b954

Reviewer: Frédéric Chapoton

Issue created by migration from https://trac.sagemath.org/ticket/18360

[rwst]

Description changed:

--- 
+++ 
@@ -8,3 +8,11 @@
 ```
 the `f(x)` is expanded into a mutable matrix, but this prevents evaluation because Pynac is called by `Expression._sub_` and wants to do hashes on the matrix for comparison reasons. However, for the purpose of subtraction dealing with a mutable matrix is completely unnecessary. So a solution would be to turn mutable objects immutable before doing arithmetics in `Expression` or in Pynac.
 
+While the most elegant solution to the specific problem would be an expression converter, applied before calling Pynac's `relational_to_bool` this will not solve other instances like
+
+```
+sage: f(x)=matrix(ZZ, 8)   
+sage: f(x)*1
+...
+TypeError: mutable matrices are unhashable
+```

[rwst]

comment:2

This is quite funny. By chance I hit on the real bug with this issue (note that f(x)+1 and f(x)^1 will work) in Pynac. No fiddling with mutable/immutable necessary it seems. Investigating.

[rwst]

Description changed:

--- 
+++ 
@@ -5,14 +5,11 @@
 sage: bool(f(x)-f(x)==0)
 ...
 TypeError: mutable matrices are unhashable
-```
-the `f(x)` is expanded into a mutable matrix, but this prevents evaluation because Pynac is called by `Expression._sub_` and wants to do hashes on the matrix for comparison reasons. However, for the purpose of subtraction dealing with a mutable matrix is completely unnecessary. So a solution would be to turn mutable objects immutable before doing arithmetics in `Expression` or in Pynac.
-
-While the most elegant solution to the specific problem would be an expression converter, applied before calling Pynac's `relational_to_bool` this will not solve other instances like
-
-```
-sage: f(x)=matrix(ZZ, 8)   
 sage: f(x)*1
 ...
 TypeError: mutable matrices are unhashable
 ```
+
+a Pynac bug manifests, see https://github.com/pynac/pynac/issues/34
+
+This ticket should provide doctests, it depends on pynac-0.4.2/0.3.9.2.

[rwst]

Dependencies: pynac-0.3.9.2

[rwst]

Upstream: Fixed upstream, in a later stable release.

[rwst]

comment:4

Critical because it prevents crashing (#14211).