Can only forget one GenericDeclaration at a time for some reason

[kcrisman]

From ​this thread:

sage: var('m')
m
sage: var('n')
n
sage: assume(n, 'integer'); assume(m, 'integer')
sage: sin(n*pi).simplify()
0
sage: sin(m*pi).simplify()
0
sage: forget()
sage: sin(m*pi).simplify()
0
sage: sin(n*pi).simplify()
sin(pi*n)

The problem seems to lie in the last few lines of _forget_all in sage.symbolic.assumptions.py, where for some reason the loop isn't doing what it should.

[kcrisman]

Unfortunately, #1163 doesn't fix this either.

[kcrisman]

The problem is actually because list removal is called in GenericDeclaration?.forget(), so this is about behavior of list iteration in Python when you remove elements:

>>> L=[1,2,3,4]
>>> for x in L:
...     L.remove(x)
...     x
...     L
... 
1
[2, 3, 4]
3
[2, 4]

So this piece of code is apparently using the wrong/un-Pythonic way of removing items from a list.

[kcrisman]

Based on 4.2

[kcrisman]

This patch should fix the issue. In fact, it's only because #7084 finally allows more than one such declaration that this bug ever showed up!

[jason]

Great catch. Doctests and documentation passes. Good work.