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Notes

This license is found in several components in TeXLive, mostly in components originating from BibTeX.

The FSF's rationale regarding the LaTeX Project Public License 1.2 applies to this specific scenario: http://www.fsf.org/licensing/licenses/index_html#GPLIncompatibleLicenses

 This license contains complex and annoying restrictions on how to
 publish a modified version, including one requirement that falls
 just barely on the good side of the line of what is acceptable: that
 any modified file must have a new name.
 The reason this requirement is acceptable for LaTeX is that TeX has
 a facility to allow you to map file names, to specify “use file bar
 when file foo is requested”. With this facility, the requirement is
 merely annoying; without the facility, the same requirement would be
 a serious obstacle, and we would have to conclude it makes the
 program non-free.

Thus, this license (and its variants) are free but GPL incompatible, but only on TeX files. If this license were applied to unrelated code, it would almost certainly be non-free due to the renaming restriction.

Original License Text

        % Copyright (C) 1985, all rights reserved.
        % Copying of this file is authorized only if either
        % (1) you make absolutely no changes to your copy, including name, or
        % (2) if you do make changes, you name it something other than
        % btxbst.doc, plain.bst, unsrt.bst, alpha.bst, and abbrv.bst.
        % This restriction helps ensure that all standard styles are identical.
        % The file btxbst.doc has the documentation for this style.

Variants

There are several variants of the Bibtex license which, while worded differently, evaluate to the same meaning.

%%%     license         = "Unlimited copying and redistribution of this
%%%                        file are permitted as long as this file is
%%%                        not modified. Modifications are permitted,
%%%                        but only if the resulting file is not named
%%%                        path.sty regardless of case",
%% This file may be modified for individual use.
%%
%% A modified version of this file may not be
%% distributed under its original name.