Re: ## not permitted in the presence of &.

[Wolfgang Schuster <schuster.wolfgang@gmail.com>]

Am 14.07.2012 um 17:20 schrieb Eythan Weg:

> Hi,
> 
> I run into trouble with  mkiv   (ConTeXt  ver: 
> 2012.05.30 11:26 MKIV  fmt: 2012.7.7  int:
> english/english).
> 
> A small example:
> \starttext
> \def\c#1#2{\halign{##\hfil&##\cr\bf#1&\quad#2\cr}}
> \c{11}{this}
> \stoptext 
> 
> gives and  error: 
> 
> ! Only one # is allowed per tab.
> 
> This runs error-free  under mkii (pdftex, xetex), plain tex
> (luatex, xetex), latex (luatex) etc.
> 
> Would someone comment? 

The error message is not very helpful in this case because the & is the problem.

In MkIV we made a while ago _, ^ and & normal characters in the text and you
don’t have to write \_ (or \textunderscore) etc. to use them. A side effect of this
was that \halign can’t be used anymore in a document (which isn’t recommended
anyway because ConTeXt has other high level table interfaces), when you now
want to use \halign to create a command put it in a \unprotect … \protect block
or use the \aligntab (it’s a LuaTeX primitive with the same meaning as &).

\unprotect

\def\one#1#2{\halign{##\hfil&##\cr\bf#1&\quad#2\cr}}

\protect

\def\two#1#2{\halign{##\hfil\aligntab##\cr\bf#1\aligntab\quad#2\cr}}

\starttext

\one{underscore}{_}

\two{ampersand}{&}

\stoptext

BTW: \c is already defined as a accent macro.

Wolfgang
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