As described in the \hyperref[copyleft-definition]{earlier general discussion

  of copyleft}, strong copyleft licenses such as the GPL seek to uphold

software freedom via the copyright system.  This principle often causes

theoretical or speculative dispute among lawyers, because ``the work'' ---

the primary unit of consideration under most copyright rules -- is not a unit

of computer programming. In order to determine whether a ``routine'' an

``object'', a ``function'', a ``library'' or any other unit of software is

part of one ``work'' when combined with other GPL'd code, we must ask a

question that copyright law will not directly answer in the same technical

terms.

consider the matter of combined and/or derivative works --- a concept that we must

understand fully before considering GPLv2~\S\S2--3\@.  At least under USA

Software licensing in general, relies critically on the concept of

``derivative work'' since software that is ``independent,'' (i.e., not

``derivative'') of Free Software need not abide by the terms of the

applicable Free Software license. As much is required by \S~106 of the

Copyright Act, 17 U.S.C. \S~106 (2002), and admitted by Free Software

licenses, such as the GPL, which (as we have seen) states in GPLv2~\S0 that ``a

`work based on the Program' means either the Program or any derivative

work under copyright law.'' It is being a derivative work of Free Software

that triggers the necessity to comply with the terms of the Free Software

license under which the original work is distributed. Therefore, one is

left to ask, just what is a ``derivative work''? The answer to that

question differs depending on which court is being asked.

The analysis in this chapter sets forth the differing definitions of

established definition of derivative work for software is the

abstraction, filtration, and comparison test (``the AFC test'') as

created and developed by the Second Circuit. Some circuits, including

the Ninth Circuit and the First Circuit, have either adopted narrower

versions of the AFC test or have expressly rejected the AFC test in

to adopt any definition of derivative work for software.

As an introductory matter, it is important to note that literal copying of

a significant portion of source code is not always sufficient to establish

that a second work is a derivative work of an original

program. Conversely, a second work can be a derivative work of an original

program even though absolutely no copying of the literal source code of

the original program has been made. This is the case because copyright

protection does not always extend to all portions of a program's code,

while, at the same time, it can extend beyond the literal code of a

program to its non-literal aspects, such as its architecture, structure,

sequence, organization, operational modules, and computer-user interface.

\section{The Copyright Act}

of a derivative work of software. However, the applicable provisions do

provide some, albeit quite cursory, guidance. Section 101 of the Copyright

Act sets forth the following definitions:

\begin{quotation}

A ``computer program'' is a set of statements or instructions to be used

directly or indirectly in a computer in order to bring about a certain

result.

A ``derivative work'' is a work based upon one or more preexisting works,

such as a translation, musical arrangement, dramatization,

fictionalization, motion picture version, sound recording, art