Re: Combining proprietary code and GPL for in-house use

[Richard Stallman <rms@gnu.org>]

    My difficulty with this argument is that an owner of the copy of the GPL
    library has a wide right to make a derivative work on the owner's computer
    by virtue of the GPL and/or a more limited right in the U.S. by virtue of
    section 117 of the U.S. Copyright Act.

In the scenario we were talking about, the addition is released
publicly *as* an addition.  If it were just made and used privately,
you might be right--in any case, that would be a different question.