incorrect handling of constant term when creating power series

[gh-DaveWitteMorris]

As reported by user TBK in ​this sage devel thread, the series method sometimes returns an incorrect power series in which a constant term has erroneously been multiplied by a power of x:

    sage: ((1-sqrt(1-x))/x + 0).series(x,3)
    1/2 + 1/8*x + 1/16*x^2 + Order(x^3)  # correct
    sage: ((1-sqrt(1-x))/x + 123).series(x,3)
    123*x^(-1) + 1/2 + 1/8*x + 1/16*x^2 + Order(x^3)  # wrong !!!

[gh-DaveWitteMorris]

Diagnosis: Laurent polynomials are stored as a pair consisting of a polynomial and an offset n that represents multiplication by x^n. Pynac's add::useries method fails to account for the offset when adding the constant term to a power series.

This should be easy to fix, so I should be able to upload a PR soon.

[gh-DaveWitteMorris]

The PR fixes the bug by moving the handling of the constant term to the initialization phase, where the offset is known to be 0.

With this PR applied, we get the correct result for the example in the ticket description:

sage: ((1-sqrt(1-x))/x + 123).series(x,3)
247/2 + 1/8*x + 1/16*x^2 + Order(x^3)

As a doctest, the PR uses the following very simple example that was also giving a wrong answer:

sage: (x^(-1) + 1).series(x,1)  # wrong answer !!!
2*x^(-1) + Order(x)

My recent pynac patches had merge conflicts, but I rebased this one on #31554 (hence the dependency), so I think it should merge cleanly. Only the last 3 commits come from this ticket ("trac 31645 handling ...", "add doctest", and "increment pynac patch level").

This is a critical bug, so I hope it can be merged in 9.3.

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Last 10 new commits:

​b6f9170	increment package version
​96b0f8b	add 32-bit known bug tags
​ed6cac2	Merge branch 'public/31554' of git://trac.sagemath.org/sage into t/31479/public/31479v2
​2364860	build/pkgs/pynac/package-version.txt: Bump patch level
​b0b074e	Merge #31479
​eeb6cc2	build/pkgs/pynac/package-version.txt: Bump patch level
​04d0090	Merge branch 'public/31479v2' of git://trac.sagemath.org/sage into t/31554/public/31554
​74bcb8a	trac 31645 handling of constant term in series
​81e530b	add doctest
​a135bbb	increment pynac patch level

[tscrim]

LGTM.

[gh-DaveWitteMorris]

Thanks!

[gh-DaveWitteMorris]

Rebased on #31679 (which replaced #31554), and updated the patch version again.

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New commits:

​3fde2ac	trac 31479: pynac's poly_mul_expand
​5501577	add doctests
​6bf6ec0	increment package version
​296d4c7	add 32-bit known bug tags
​ef84adf	Merge branch 'public/31554' of git://trac.sagemath.org/sage into #31679polymulexpand
​2b88eb5	fix doctest for 32-bit
​9aec019	trac 31645 handling of constant term in series
​89d65ff	add doctest
​1da52de	increment pynac patch level again