"Stricts _"

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