Original sentence: According to some, the EEE strategy is also used by some free software.Comparison of GNU/Linux and previous UNIX operating systems has led some to conclude that GNU/Linux has embraced, extended and extinguished most of them as of today, most prominently SCO UNIX, the former market leader of UNIX on IBM PC compatible computers.Free software developers have implemented numerous nonstandard (and subject to the GPL and other licenses) extensions in free compilers, libraries, utilities, and on-disk formats and it is sometimes difficult to adopt only part of the solution.Remaining ones like Solaris are struggling and subsist merely because vendors primarily draw revenue from computer hardware.Since free software is not public domain, UNIX vendors must write their own implementations of extensions, or allow their customers the same freedoms as the free software if they wish to copy the source code of the extensions outright.The Linux kernel requires C compiler extensions only present in the GNU C compiler, just as Windows code requires extensions originally only present in the Microsoft C compiler