[GiNaC-list] factor() hangs on a simple expression, randomly

[Vitaly Magerya]

Hi, folks. I have this problem:

    $ ginsh
    ginsh - GiNaC Interactive Shell (GiNaC V1.7.2)
      __,  _______  Copyright (C) 1999-2017 Johannes Gutenberg University Mainz,
     (__) *       | Germany.  This is free software with ABSOLUTELY NO WARRANTY.
      ._) i N a C | You are welcome to redistribute it under certain conditions.
    <-------------' For details type `warranty;'.

    Type ?? for a list of help topics.
    > factor((2*x-l)^2*(-1+6*x-l)*(-1+3*x-l)*(-2+4*x-l)*(1+4*x-l)*(4*x-l)*(2+4*x-l));

At this point ginsh either hangs (80% of cases), or gives the correct
result (20% of cases).

As you can see the expression is already factored, so a hang is pretty
unexpected here. Note that if you'll drop two of the factors from that
expression, GiNaC won't hang anymore.

Here are the cumulative timings you'll typically see via 'perf top'
during this hang:

    91.54% libginac.so.6.0.2  [.] GiNaC::(anonymous namespace)::factor_multivariate
    81.32% libginac.so.6.0.2  [.] GiNaC::(anonymous namespace)::hensel_multivar
    79.67% libginac.so.6.0.2  [.] GiNaC::(anonymous namespace)::multivar_diophant
    79.67% libginac.so.6.0.2  [.] GiNaC::(anonymous namespace)::univar_diophant
    79.67% libginac.so.6.0.2  [.] GiNaC::(anonymous namespace)::multiterm_eea_lift
    72.77% libginac.so.6.0.2  [.] GiNaC::(anonymous namespace)::factor_sqrfree
    66.18% libginac.so.6.0.2  [.] GiNaC::(anonymous namespace)::eea_lift
    64.28% libginac.so.6.0.2  [.] GiNaC::factor
    26.99% libginac.so.6.0.2  [.] GiNaC::(anonymous namespace)::operator*
    23.67% ginsh              [.] yyparse
    23.67% ginsh              [.] f_factor
    16.91% libginac.so.6.0.2  [.] cln::cl_MI::~cl_MI
    15.26% libginac.so.6.0.2  [.] cln::cl_I::~cl_I
    13.66% libginac.so.6.0.2  [.] cln::cl_MI::cl_MI
    13.55% libginac.so.6.0.2  [.] cln::cl_RA::~cl_RA

So, any ideas on the nature of this problem, and how to fix it?