What do you expect here to pass as argument to the oldfact command?
>
I will pass something like n, 5, 10 etc.

>
> This works here:
>
> \defineenumeration
>   [proof]
>   [closesymbol={\mathematics{\square}}]
>
> \starttext
>
> \startproof
> \samplefile{lorem}
> \stopproof
>
> \stoptext
>

Thanks. I will test this again.
-- 
Respect,
Shiv Shankar Dayal