Open/Free Licenses (was: propose: `cypherpunks license' (Re: Wanted: Twofish source code))

[Richard Stallman]

    Richard, as I'm sure you're aware, this whole maze of license compatibility
    can initially be quite confusing.  What is the nature of the
    incompatibility between MPL and GPL?

The GNU GPL is a true copyleft: it says that modified versions of the
program must be distributed *as a whole* under the GPL.  Therefore, to
add another file to a modified version of the program, the criterion
is this: do you have permission to include that text in something
covered by the GNU GPL?

The MPL is incompatible with the GPL because it does not give
permission for this.  The LGPL explicitly gives permission for this,
and that's how it manages to be compatible with the GPL.

Any real copyleft would raise the same issue as the GPL.