Sept 3, 2003 Free Software Licensing in Context GPL: Source distribution
The GPL requires that source code be available to people who get binaries. One way to do that is to include it with the binaries:
(3)(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...
Source code is defined:
The source code for a work means the preferred form of the work for making modifications to it.

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 A mechanical hound, which is definately not the unofficial mascott of the GPL Compliance Lab. Copyright © 2003, Free Software Foundation. Verbatim copying permitted provided this notice is preserved.