FWIW:

```
'expr' is a program used in ancient shell code to do math. In Posix shells like bash, use
$(( expression )). In bash and ksh93, you can also use '(( expression ))' or 'let
expression' if you don't need to use the result in an expansion.

The backquote (`) is used in the old-style command substitution, e.g. foo=`command`. The
foo=$(command) syntax is recommended instead. Backslash handling inside $() is less
surprising, and $() is easier to nest. See http://mywiki.wooledge.org/BashFAQ/082
```

I wouldn't worry about it now, but I'd like to change these when I rebase the bash branch.