[GiNaC-list] Collect() with a list of variables doesn't work in ginsh

[Vitaly Magerya]

Hi, all. In section 5.7.2 of the docs (Expanding and collecting)
there is this example of ginsh usage:

     > a=expand((sin(x)+sin(y))*(1+p+q)*(1+d));
     > collect(a,{p,q});

The docs say I should get this:

     d*sin(x)+(d*sin(x)+sin(y)+d*sin(y)+sin(x))*p

... but actually:

     cannot modify multiply referenced object

This is with GiNaC V1.7.6, V1.7.8, and the latest commit of
V1.8.0.

How do I make this work?