On Thu, 2005-01-27 at 13:20 +0100, Oliver Bandel wrote:

> # match Foo with
>    _Foo -> "foo"
>   |Bar -> "bar";;
> Warning: this match case is unused.
> - : string = "foo"

Here, you're matching the value Foo against a catch-all pattern with a
variable called _Foo.  This is equivalent to writing:

match <anything> with
   x -> "foo"
 | <anything> -> "bar"

The first case matches any value.