escaping needed for negative ${parameter:offset}

[Dan Jacobson]

The man page should warn about escaping needed
$ a=0123456789;echo ${a:-3}
0123456789
$ a=0123456789;echo ${a:$((-3))}
789
$ a=0123456789;echo ${a:(-3)}
789
$ a=0123456789;echo ${a: -3}
789
to avoid conflicting with ${parameter:-word}