+=

Is there any equivalent to +=, or *= in max?

x:+y
x:*y

There's a section in the docs somewhere with all the options

var1:+var2
var1:*var2
var1:-var2
var1:/var2

I have no idea what += and *= are but you can say
value:+2
which is the same as
value=value+2
The same way you can say value:*2

Cool! Thanks!

+= just means the same as a=a+b - add something to the value and store it in the original variable. Same for *=