Macro as a counter? Options

[katkt]

I'm not sure if something I'm trying to do is possible or not. It might require programmable triggers.

Essentially, I want to count how many times the mud I play sends me a particular message. So the first time I get the message:
* You think your foo skill has improved. *

I want to set some macro to 1. And then the next time I get that message, I want to set the macro to 2. And so on.

I can make a trigger set a macro, but I can't figure out how to do the addition. I tried
##set macro "%%1" %%1+1

But of course, that just set the variable <foo> whose values was <0> to <foo+1>.
Is there currently any way to get any sort of evaluation to happen in the set macro command? So that I could do some calculation based on the current value of the variable <foo> in order to figure out what the new value of the variable <foo> should be?