Errata:

   In the paper:

    Felsenstein, J.  1990.  Counting phylogenetic invariants in some simple
       cases.  Journal of Theoretical Biology  152: 357-376.

The last term in equation (24) (Lake's first invariant) should read

         -(1/3) P_{xyzy}

instead of

         -(1/3) P_{xyzx}