Clean up MPolynomialRing_generic.completion

[jdemeyer]

In src/sage/rings/polynomial/multi_polynomial_ring_generic.pyx, the method completion() constructs a PowerSeriesRing either from variable names or from an polynomial ring element. The latter must be changed for #10483. Moreover, the error checking also needs to be improved.

CC: @tscrim

Component: algebra

Author: Jeroen Demeyer

Branch/Commit: 52e13d8

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/23377

[jdemeyer]

Author: Jeroen Demeyer

[jdemeyer]

Branch: u/jdemeyer/clean_up_mpolynomialring_generic_completion

[jdemeyer]

Commit: 8108128

[jdemeyer]

New commits:

8108128

Clean up MPolynomialRing_generic.completion

[tscrim]

comment:4

The doc for completion is no longer correct (it mentions p). Also, do we want to support R.completion('x,y')?

[jdemeyer]

comment:6

Replying to @tscrim:

Also, do we want to support R.completion('x,y')?

Not sure. So far, there doesn't seem to be a need.

[sagetrac-git]

Changed commit from 8108128 to 7cfb4af

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

92bafe2	Merge tag '8.1.beta2' into t/23377/clean_up_mpolynomialring_generic_completion
7cfb4af	Fix doc of completion()