I'd like to POSIX-ly add two numbers together in my shell script. I know I can do this:

```
a=1
b=2
sum=$((a+b))

```

But if I try that with ``` (i.e. `sum=`(a+b)``), it doesn't work; and the `$( ... )` syntax isn't POSIX. I can also pipe it to `bc` or `expr`, but I'd prefer not to, if I can help it.