Author: Unknown
Subject: Why is this an invalid expression?
Date: Saturday, 17 Oct 2020, 11:18:39

$ /c/cs323/Hwk3/mcBash
...
(7)$ ${e=${f-${_}}}
Invalid expansion
(7)$ ${f-${_}}
>> /c/cs323/Hwk3/mcBash

Hi, could you explain why is this an invalid expression?