not all fractions get converted with ex.fraction()

[rws]

sage: a = var('a')
sage: b = (1/a).fraction(QQ)
...
TypeError: unsupported operand parent(s) for '/': 'Univariate Polynomial Ring in x over Rational Field' and 'Univariate Polynomial Ring in a over Rational Field'

The 1 apparently gets coerced into the x-ring, so we need to ensure equal rings somehow.

[rws]

New commits:

​b46d1df

17736: not all fractions get converted with ex.fraction()

[rws]

Passes all patchbot tests.

[malb]

Looks good to me. A potential feature request could be to allow the user to specify the ordering of variables, but perhaps that's overkill?

[rws]

I'm not so familiar with algebra. Would you suggest specific orderings or is there a convention?

[malb]

Hi, sorry for not being clear:

I merely meant the ordering of variable (names), where you currently call sort(). But as I said: it's an optional feature request which could go into a separate ticket.

Of course, arbitrary term ordering ("lex" and "deglex" etc.) could also be considered. But I'd say a user should do the conversion by hand perhaps in this case.

[rws]

Replying to malb:

I merely meant the ordering of variable (names), where you currently call sort(). But as I said: it's an optional feature request which could go into a separate ticket.

Ah but yes, I agree. Thanks for the review.