What is _SAGE_VAR_(0)?

Using SageMath 7.4, I do the following:

sage: var('u R omega')
sage: hs = (1+u^R).series(u==omega, 2); hs
(omega^R + 1) + (R*omega^R/omega)*(-omega + u) + Order((omega - u)^2)
sage: hs.subs({omega^R: -1}).simplify()
R*(omega - u)/omega + Order((omega - u)^2) + _SAGE_VAR_(0)


I am wondering what _SAGE_VAR_(0) shall be and how to get rid of it.

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That is a leftover from Maxima's translation back to Sage. Somehow it didn't get translated. Looking at the precise expression hs, I suspect probably _SAGE_VAR_(0) is what came out of omega^R + 1, which then becomes zero - but a symbolic zero. So we are probably missing a translation in series (or in general) when a symbolic zero is brought back from Maxima. I don't have a solution for that but those with time in the know should be able to get it from that.

Edit: This is definitely known and apparently even a fix is available, we need someone to review and test it.

See https://trac.sagemath.org/ticket/20846 and https://trac.sagemath.org/ticket/17400 which needs review.

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