This code example makes no sense to me.

What does an Itcl 3 programmer expect a
command [a::a] to do in that context?

Best I can guess this is an ill-conceived
attempt to reach into and abuse the particular
implementation of Itcl 3, which Itcl 4 does not
reproduce.

I could be wrong though, I'm not a user of Itcl 3,
and don't know the full set of programming
expectations its users have.