GNU GENERAL PUBLIC LICENSE
Version 2, June 1991

Copyright (C) 1989, 1991 Free Software Foundation, Inc.
51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

Preamble

The licenses for most software are designed to take away your
freedom to share and change it. By contrast, the GNU General Public
License is intended to guarantee your freedom to share and change
free software--to make sure the software is free for all its users.
This General Public License applies to most of the Free Software
Foundation's software and to any other program whose authors commit
to using it. (Some other Free Software Foundation software is
covered by the GNU Library General Public License instead.)
You can apply it to your programs, too.

[完整文本较长，这里只贴核心部分，完整内容请见：https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt]

TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

0. This License applies to any program or other work which contains
a notice placed by the copyright holder saying it may be distributed
under the terms of this General Public License. The "Program", below,
refers to any such program or work, and a "work based on the Program"
means either the Program or any derivative work under copyright law:
that is to say, a work containing the Program or a portion of it,
either verbatim or with modifications and/or translated into another language.
(Hereinafter, translation is included without limitation in the term
"modification".)

1. You may copy and distribute verbatim copies of the Program's
source code as you receive it, in any medium, provided that you
conspicuously and appropriately publish on each copy an appropriate
copyright notice and disclaimer of warranty; keep intact all the
notices that refer to this License and to the absence of any warranty;
and give any other recipients of the Program a copy of this License
along with the Program.

2. You may modify your copy or copies of the Program or any portion
of it, thus forming a work based on the Program, and copy and
distribute such modifications or work under the terms of Section 1
above, provided that you also meet all of these conditions:

  a) You must cause the modified files to carry prominent notices
  stating that you changed the files and the date of any change.

  b) You must cause any work that you distribute or publish, that in
  whole or in part contains or is derived from the Program or any
  part thereof, to be licensed as a whole at no charge to all third
  parties under the terms of this License.

  c) If the modified program normally reads commands interactively
  when run, you must cause it, when started running for such
  interactive use in the most ordinary way, to print or display an
  announcement including an appropriate copyright notice and a
  notice that there is no warranty (or else, saying that you provide
  a warranty) and that users may redistribute the program under
  these conditions, and telling the user how to view a copy of this
  License. (Exception: if the Program itself is interactive but
  does not normally print such an announcement, your work based on
  the Program is not required to print an announcement.)

3. You may copy and distribute the Program (or a work based on it,
under Section 2) in object code or executable form under the terms
of Sections 1 and 2 above provided that you also do one of the
following:

  a) Accompany it with the complete corresponding machine-readable
  source code, which must be distributed under the terms of Sections
  1 and 2 above on a medium customarily used for software interchange; or,

  b) Accompany it with a written offer, valid for at least three
  years, to give any third party, for a charge no more than your
  cost of physically performing source distribution, a complete
  machine-readable copy of the corresponding source code, to be
  distributed under the terms of Sections 1 and 2 above on a medium
  customarily used for software interchange; or,

  c) Accompany it with the information you received as to the offer
  to distribute corresponding source code. (This alternative is
  allowed only for noncommercial distribution and only if you
  received the program in object code or executable form with such
  an offer, in accord with Subsection b above.)

4. You may not copy, modify, sublicense, or distribute the Program
except as expressly provided under this License. Any attempt
otherwise to copy, modify, sublicense or distribute the Program is
void, and will automatically terminate your rights under this License.

5. You are not required to accept this License, since you have not
signed it. However, nothing else grants you permission to modify or
distribute the Program or its derivative works. These actions are
prohibited by law if you do not accept this License. Therefore, by
modifying or distributing the Program (or any work based on the
Program), you indicate your acceptance of this License to do so.

[...]

(全文请见 https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt )
