To achieve that goal, GPLv2~\S2 first sets forth that the rights of

redistribution of modified versions are the same as those for verbatim

copying, as presented in GPLv2~\S1.  Therefore, the details of charging money,

keeping copyright notices intact, and other GPLv2~\S1 provisions are intact

here as well.  However, there are three additional requirements.

The first (GPLv2~\S2(a)) requires that modified files carry ``prominent

notices'' explaining what changes were made and the date of such

changes. This section does not prescribe some specific way of

marking changes nor does it control the process of how changes are made.

Primarily, GPLv2~\S2(a) seeks to ensure that those receiving modified

versions know the history of changes to the software.  For some users,

warranty disclaimer, modification indications and a pointer to the license,

even in modified versions.  The requirement is relatively simple, and relates

to an important policy goal of copyleft: downstream users should be informed

of their rights.  Its implications and details are straightforward and

simple.

Meanwhile, GPLv2~\S2(b) requires careful and extensive study.  Its four short lines embody

the some of the essential legal details of ``share and share alike''.  These 46 words are

considered by some to be the most worthy of careful scrutiny because

GPLv2~\S2(b), and they

can be a source of great confusion when not properly understood.