Re: forwarded message from Jeff Licquia
On Fri, 2002-07-19 at 15:38, Boris Veytsman wrote:
> > Date: Fri, 19 Jul 2002 14:45:35 +1200
> > From: Nick Phillips <nwp@nz.lemon-computing.com>
>
> > If on the other hand the LaTeX license allowed us to do "whatever we like,
> > so long as anything that's called LaTeX produces identical output (documents)
> > to unmodified LaTeX for all valid input" then we could butcher LaTeX to use
> > a not-quite-TeX that had the bug fixed, and still produced identical output.
>
> This is impossible due to a nice rendition of Goedel
> theorem. Basically it says that if your language is complex enough
> (well, if you can program a Turing machine in your language), then you
> cannot make a program that can in finite time automatically prove or
> disprove that a pair of programs gives identical results for all valid
> inputs. Since TeX the language is Turing-complete (note that TeX the
> engine is not, because it has limitation for the number of registers),
> you never can prove that two LaTeXs give identical output for all
> valid inputs.
>From a licensing perspective, this isn't a problem. We can, for
example, reference a test suite in the license (doesn't TeX do this?),
or perhaps require that any demonstrated inconsistency must be fixed.
This transfers the onus onto the person claiming a license violation;
that person must provide definitive proof of an inconsistency not
allowed by the license.
You do have to be careful about your wording, however, so that the
intent is carefully spelled out. For example, you might want to allow
bug fixes without renaming; in that case, you'd want to define "bug fix"
so the "inconsistency" of a fix isn't used as evidence of a license
violation.
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