>>>>> On Wed, 17 Jun 2020 08:24:21 +0000, Feng Feng <f.feng@outlook.com> said:

    FF> Hi all, I got a strange problem about collect_common_factors,
    FF> the problem can be reproduced by the following example: ------

    The trick with your expression is that the common factor (and thus
  the entire expression) is equal to 0. I have sent a patch to fix this:

https://www.ginac.de/pipermail/ginac-devel/2020-June/002473.html

  Best wishes,
        Vladimir
--
Vladimir V. Kisil                 http://www.maths.leeds.ac.uk/~kisilv/
  Book:     Geometry of Mobius Transformations     http://goo.gl/EaG2Vu
  Software: Geometry of cycles          http://moebinv.sourceforge.net/
  Jupyter: https://github.com/vvkisil/MoebInv-notebooks