hold=True does not always prevent evaluation

[mezzarobba]

sage: x = SR.var(x)
sage: x + 1/3
x + 1/3
sage: (x + 1/3).power(2, hold=True)
(x + 1/3)^2
sage: (x + 1/3).power(2, hold=True).mul(2, hold=True) # <<< evaluates!
2*(1/9*(3*x + 1)^2)
sage: (x + 1/3).mul(2, hold=True)
2*(x + 1/3)

Reported as pynac/pynac#262

Depends on #10035

Upstream: Fixed upstream, in a later stable release.

CC: @DaveWitteMorris @slel

Component: symbolics

Author: Frédéric Chapoton

Branch/Commit: u/chapoton/23304 @ 277efed

Issue created by migration from https://trac.sagemath.org/ticket/23304

[rwst]

comment:1

See also #21754 and #21758.

[rwst]

Upstream: Reported upstream. Developers acknowledge bug.

[rwst]

Description changed:

--- 
+++ 
@@ -10,3 +10,5 @@
 sage: (x + 1/3).mul(2, hold=True)
 2*(x + 1/3)
 ```
+
+Reported as https://github.com/pynac/pynac/issues/262

[rwst]

Dependencies: #10035

[rwst]

comment:3

Operations like this are now better done in a hold context. With #10035:

sage: hold.start()
sage: x + 1/3
x + 1/3
sage: (x + 1/3)^2
(x + 1/3)^2
sage: (x + 1/3)^2*2
2*(x + 1/3)^2
sage: (x + 1/3)*2
2*(x + 1/3)

[rwst]

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

[slel]

comment:4

The upstream fix in Pynac was merged in Pynac 0.7.11.

The upgrade to Pynac 0.7.11 was done in #23820, merged in Sage 8.1.beta6.

In the present ticket we should add a doctest.