[NTG-context] Grouping with \dosingleempty

[Taco Hoekwater]

Aditya Mahajan wrote:
> Hi,
>   I am confused why the following definition does not work as expected.
> 
> \def\PM{\dosingleempty\doPM}
> \def\doPM[#1]{\iffirstargument P_{M_{#1}}
>    \else P_{M_t}\fi}
> 
> \starttext
> Why is $\widetilde \PM[T]$ not the same as $\widetilde P_{M_T}$
> \stoptext

The argument to \widetilde cannot be a macro that requires
arguments, that just won't work. If you want the accent on
top of a single character, you have to give it a single
argument. It is possbile to do so, like this:

   \def\PM{P\dosingleempty\doPM}
   \def\doPM[#1]{\iffirstargument _{M_{#1}}\else _{M_t}\fi}
   \let\normalwidetilde\widetilde
   \def\widetilde{\expandafter\normalwidetilde}

But it is better to create a new set of macros \widetildePM.

Cheers,
Taco