[texhax] What's legal about \halign and backward braces?

[Uwe Lueck]

I wrote a few minutes ago:
>> Doug McKenna <doug at mathemaesthetics.com> writes:
>>
>>> \halign{}
>>>
>>> This unexpectedly (to those versed in usual computer language syntax)
>>> leads to a runawy preamble error, because the '}' doesn't end the
>>> command's parsing; a # and then a \cr must happen first, and they're
>>> missing.
>>
Donald Arseneau: >> My immediate reaction is surprise.
>
> Indeed, I do not get an error with \halign{}!
> Doug, please present that minimal file that produces
> the error you are reporting.

I had only tried on the command line. In that case, on the next 
lines \end doesn't have any effect, but \bye throws an error.
Indeed a file containing *nothing but*

    \halign{}

throws the error -- at its end.

The file can contain whatever, it seems, apart from special
commands like \bye, TeX patiently waits for continuation
of the preamble line in the file.

On the other hand, a *file* consisting of nothing but

    \halign{}#{\cr}\end

is accepted.

Cheers,

    Uwe.