Giving a formula a specific name in ConTeXt

I would like to name a equation with a specific name ; for example, something like ($A_1$) ; I tried this structure from the wiki :

startformula
letdoplaceformulanumberempty
c^2 = a^2 + b^2   leqno{formulanumber}
stopformula

and changed formulanumber with $A_1$, but nothing displays. I also naively tried to replace leqno with tag as I saw on the forum it is used in LaTeX, but in ConTeXt it gave me an "undefined control sequence" error.

Is there a hack to do this in ConTeXt ?

starttext
startplaceformula[title=$A_1$,reference=A1]
startformula
c^2 = a^2 + b^2   
stopformula
stopplaceformula
However, in[A1] doesn't work. 
%%% Use !, another kerning or whatever you need
%%% to remove the space after the reference
In such a case you could manually write in{$A_1$}[A1]kern-1ex.
stoptext