Small puzzle

[mjd-perl-pm@plover.com (Mark Dominus)]

I have a bunch of nodes, which are abstract objects.  Nodes have a
'parent' method, which returns the parent node, if there is one, or
'undef' if not.  I can also compare two nodes to see if they are
equal.  No node has more than one parent.

I want to write a function which, given a list of nodes, returns the
youngest common ancestor of all the nodes in the list, or undef if
there is no common ancestor.  For example, given nodes organized like
this:


                A               M
               / \              |
              B   C             N
             / \  |
            D   E F
                |
                G

The common ancestor of D and G is B;
the common ancestor of D and E is B;
the common ancestor of D and B is B;
the common ancestor of D and A is A;
the common ancestor of D and F is A;
the common ancestor of G and E is E;
the common ancestor of D and M is undefined
the common ancestor of N and M is M.

My function to do this is 18 lines long.  This is way too much code.
I am sure there must be a simpler solution.  Can anyone think of a
simpler solution?

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