Variable nomenclature...

You can not perform arithmetic operations on variables that begin with a numeral.
This will be let by the Command Prompt, though it should not in my opinion (variables in general should never begin with a numeral).

set 8var=3

But this one fails:

set/a 8var=3

In the latter example, it is sufficient to preceed the variable name with some character for it to start working, for instance:

set/a _8var=3