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