Bug in ExpressionNice with composite variables

[egourgoulhon]

In Sage 9.6.beta2, we have

sage: from sage.manifolds.utilities import ExpressionNice
sage: u, v = var('u v')
sage: f = function('F')(u + v)
sage: f
F(u + v)
sage: ExpressionNice(diff(f, u))
d(F)/d(u + v)

So far, so good, but

sage: f = function('F')(u - v)
sage: ExpressionNice(diff(f, u))
d(F)/du - v

This bug has been reported in https://groups.google.com/g/sage-support/c/fbE0APqThEk

With the fix introduced in this ticket, the last output is now

d(F)/d(u - v)

CC: @tscrim

Component: manifolds

Author: Eric Gourgoulhon

Branch/Commit: 9b9b65b

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/33399

[egourgoulhon]

Branch: public/manifolds/expression_nice-33399

[egourgoulhon]

Commit: 70729d9

[egourgoulhon]

New commits:

70729d9

Fix bug in ExpressionNice (#33399)

[egourgoulhon]

Description changed:

--- 
+++ 
@@ -17,3 +17,9 @@
 d(F)/du - v
 ```
 This bug has been reported in https://groups.google.com/g/sage-support/c/fbE0APqThEk
+
+With the fix introduced in this ticket, the last output is now
+
+```
+d(F)/d(u - v)
+```

[mkoeppe]

Reviewer: Matthias Koeppe

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1 and set ticket back to needs_review. New commits:

9b9b65b

Remove import re from ExpressionNice (#33399)

[sagetrac-git]

Changed commit from 70729d9 to 9b9b65b