[NTG-context] Grouping with \dosingleempty

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 Can I change the definition so that $\widetilede \PM$ is same as $\widetilde P_{M_t}$ and $\widetilede \PM[T]$ is same as $\widetilde P_{M_T}$ Thanks, Aditya