math: command for absolute value: `\define\abs[1]{\lvert #1 \rvert}` throws `\lvert ->\Udelimiter`.

[Paul Menzel <paulepanter@users.sourceforge.net>]

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Dear ConTeXt folks,


I want to define `\abs` as the command for the absolute value. The
following minimal example is also attached.

        \define\abs[1]{\lvert #1 \rvert}
        \starttext
        $\lvert 2 \rvert$
        
        $\abs{2}$
        \stoptext

This does not work though.

        $ context abs
        […]
        This is LuaTeX, Version beta-0.70.1-2011051923 (rev 4277) 
         \write18 enabled.
        (abs.tex
        
        ConTeXt  ver: 2011.05.18 22:26 MKIV  fmt: 2011.5.27  int: english/english
        […]
        ! Missing $ inserted.
        
        system          > tex > error on line 1 in file abs.tex: Missing $ inserted ...
        
        1 >>  \define\abs[1]{\lvert #1 \rvert}
        2     \starttext
        3     $\lvert 2 \rvert$
        4     
        5     $\abs{2}$
        6     \stoptext
        7     
        
        <inserted text> 
                        $
        <to be read again> 
                           \Udelimiter 
        \lvert ->\Udelimiter 
                             "4 "0 "7C 
        l.1 \define\abs[1]{\lvert
                                  #1 \rvert}
        ?

How should I do that correctly? I took that example from the user’s
guide of the amsmath package (`texdoc amsldoc`).


Thanks,

Paul

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\define\abs[1]{\lvert #1 \rvert}
\starttext
$\lvert 2 \rvert$

$\abs{2}$
\stoptext

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