[GSoC] Series: Fixing incorrect limit evaluations caused due to bug in rewriting

[sachin-4099]

Fixes: #18378
Fixes: #18482
Closes #18947

Brief description of what is fixed or changed

Incorrect limit evaluation takes place because of bug in rewriting.

The function xreplace() in the rewrite() function of gruntz.py is not substituting properly.
As a result, it has been replaced by the subs() function which resolves the issue.

Other Comments

Regression Tests have been added.

Release Notes

• series
  • Replaces xreplace() with subs() in rewrite() function of gruntz.py resolving incorrect limit evaluations

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• series
  • Replaces xreplace() with subs() in rewrite() function of gruntz.py resolving incorrect limit evaluations (#19297 by @sachin-4099)

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Fixes: #18378
Fixes: #18482 
Closes #18947

#### Brief description of what is fixed or changed

**Incorrect limit evaluation** takes place because of **bug in rewriting.**

The function `xreplace()` in the `rewrite()` function of `gruntz.py` is not substituting properly.
As a result, it has been replaced by the `subs()` function which resolves the issue.

#### Other Comments
Regression Tests have been added.


#### Release Notes


<!-- BEGIN RELEASE NOTES -->
* series
  * Replaces `xreplace()` with `subs()` in `rewrite() function of gruntz.py` resolving incorrect limit evaluations 
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #19297 into master will decrease coverage by 0.010%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master    #19297       +/-   ##
=============================================
- Coverage   75.595%   75.585%   -0.011%     
=============================================
  Files          651       651               
  Lines       169545    169545               
  Branches     40016     40016               
=============================================
- Hits        128168    128151       -17     
- Misses       35762     35780       +18     
+ Partials      5615      5614        -1     

[jksuom]

Why doit()?

[sachin-4099]

Yeah actually not needed. Will change it asap. Anything else?

[jksuom]

Looks good. xreplace does not find all that should be replaced.