how to change a factory use of rfc822.Message to mhlib.Message

[John Doe]

I've never used a factory and would like to get some help.
The class
mailbox.UnixMailbox has a defination for a factory.

__init__(self, fp, factory=<class rfc822.Message>)

where fp is a file pointer. 

I don't want to use rfc822.Message. I'd like to use mhlib.Message instead.

rfc822.Message requires:
__init__(self, fp, seekable=1)

mhlib.Message requires:
__init__(self, f, n, fp=None)

where f think the is a folder name. I've seen '.'  used.
(The documentation is very unclear here). And 'n' is the seekable.

How do I call mailbox.UnixMailbox( fp ... ) with mhlib.Message?

Thanks