The penultimate paragraph in GPLv3\S2 notes that GPLv3's CCS definition does

not require source that can be automatically generated.  Many code

generators, preprocessors and take source code as input and sometimes even

have output that is still source code.  Source code should always be whatever

the original programmer preferred to modify.

GPLv3\S1's final paragraph removes any ambiguity about what should be done on

source-only distributions.  Specifically, the right to convey source code

that does not compile, does not work, or otherwise is experimental

in-progress work is fully permitted, \textit{provided that} no object code

form is conveyed as well.  Indeed, when combined with the permissions in

GPLv3\S~5, it is clear that if one conveys \textit{only} source code, one can

never be required to provide more than that.  One always has the right to

modify a source code work by deleting any part of it, and there can be no

requirement that free software source code be a whole functioning program.

\subsection{The System Library Exception}

The previous section skipped over one part of the CCS definition, the

so-called system library exception.  The ``System Libraries'' definition (and

the ``Standard Interface'' and ``Major Component'' definitions, which it

includes) are designed as part

to permit certain distribution arrangements that are considered reasonable by

copyleft advocates.  The system library exception is designed to allow

software freedom more than it would hurt proprietary software.

The system library exception has two parts.  Part (a) rewords the GPLv2

exception for clarity replacing GPLv2's words ``unless that component itself

accompanies the executable'' with ``which is not part of the Major

Component''.  The goal here is to not require disclosure of source code of

certain libraries, such as necessary Microsoft Windows DLLs (which aren't

part of Windows' kernel but accompany it) that are required for functioning

of copylefted programs compiled for Windows.

However, in isolation, (a) would be too permissive, as it would sometimes

allowing distributors to evade important GPL requirements.  Part (b) reigns

in (a).  Specifically, (b) specifies only a few functionalities that a

system library may provide and still qualify for the exception.  The goal is

to ensure system libraries are truly adjunct to a major essential operating

system component, compiler, or interpreter.  The more low-level the

functionality provided by the library, the more likely it is to be qualified

for this exception.

Admittedly, the system library exception is a frequently discussed topic of

obsessed GPL theorists.  The amount that has been written on the system

library exception (both the GPLv2 and GPLv3 versions of it), if included

herein,  could easily increase this section of the tutorial to a length

greater than all the others.