What's the rule for dependencies of 'contrib' packages?

[Nathanael Nerode <neroden@twcny.rr.com>]

'muffin' isn't allowed into 'testing' because it depends on 'jdk1.1', 
which isn't in Debian.

But 'muffin' is contrib.  I thought that 'contrib' packages were allowed 
to depend on packages not in the Debian archive.

Or is the rule that all dependencies of 'contrib' packages must be in 
'non-free', in 'contrib', or in Debian?  I didn't see any such rule.

Perhaps a clarification is in order?