Open source licenses are noted for being self-referential. The two dominant licenses at the early stage of the open source movement were GPL and BSD. GPL says the next developer cannot go proprietary, and can only go open source with the same license, namely GPL. BSD says the next developer can go proprietary, and can also go open source with any open source license, including BSD. We construct the universal space of all self-referential licenses such as GPL and BSD. We also provide a plausible explanation of why GPL and BSD stood out from other licenses as the two most natural choices for the first-generation open source developers.
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