Nodefault option and case sensitivity

[Bill Fenlason]

With flex 2.5.4 (Cygwin), I'm using "%option nodefault" to insure there are
no holes in my scanner rules.

When I specify "%option case-sensitive", there is  no warning message.

I assume that means that the default action will never be taken under any
input, no matter what the case is of any input letter.

But when I specify "%option case-insensitive" to fold lower case to upper, I
get a warning message that the default rule can be matched.

Does this mean that a hole can exist when upper and lower case are folded
even if there is no hole without the folding?

I don't understand how that can happen.  Can someone explain?

Thank you.

Bill Fenlason