add <number> to <user variable>
e.g. add 2 to n3
There are 10 user variables of this type, and their values can be obtained by the normal expansion syntax (for example $n3) in other commands. At the start of filtering, these variables all contain zero. Both arguments of the add command are expanded before use, making it possible to add variables to each other. Subtraction can be obtained by adding negative numbers.