terminology: double-rooted DAG?

I have a DAG data structure that has two special
features:


1. It always has a defined root node (like a tree), and
2. There is also a "tail" node, which is like the root
        node except at the opposite end of the graph.


I have been calling this a "doubly rooted DAG", but I
would like to know if there is a better or more
standard term for this. Also, any references to this
type of data structure would be appreciated.


Thanks,
Mark.


In case I'm doing a bad job explaining, here's an
example... "x" is the head and "y" is the tail.
x -> a
x -> b
x -> c
a -> d
b -> d
b -> y
c -> y


      /-a---d-\
x----b--/-----y
      \-c----/


markh@usai.asiainfo.com
Mark Harrison at AsiaInfo Computer Networks, Beijing China
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