APT Removing Packages *With Met* Dependancies?

[ms419]

Why must libgle3 and xscreensaver-gl be removed when I try to install a 
package which conflicts with, but provides xlibmesa-gl?

I'm experimenting with some (ugly) video drivers. I need to remove 
xlibmesa-gl and install a different libGL.so, so I used the following 
"control" file and equivs to build xlibmesa-gl-dummy:

---
Package: xlibmesa-gl-dummy
Provides: xlibmesa-gl
Conflicts: xlibmesa-gl
---
However, when I try to install xlibmesa-gl-dummy, APT insists on 
removing libgle3 and xscreensaver-gl.

---
# apt-get install xlibmesa-gl-dummy
Reading Package Lists... Done
Building Dependency Tree... Done
The following packages will be REMOVED:
  libgle3 xlibmesa-gl xscreensaver-gl
The following NEW packages will be installed:
  xlibmesa-gl-dummy
0 upgraded, 1 newly installed, 3 to remove and 0 not upgraded.
Need to get 2406B of archives.
After unpacking 7878kB disk space will be freed.
Do you want to continue? [Y/n]
---
As far as I understand, these packages' dependancies should be met ... 
Can someone spot what I'm doing wrong?

Thanks!
Jack
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