"Stricts _"

[Peter Simons]

ghci-6.3 (from CVS) shows me strictness information when I
request :info for a data type, like:

    [...] Send Mailbox   Stricts: _ [...]

I have no idea how to read that output, and it doesn't seem
to be documented in the manual either. Does the underscore
signify that (in the example above) Mailbox is a strict
value? Or is it the opposite?

Sorry if this is a dumb question, but I simply don't know
it. :-)

Peter