Hi Sarah, Representing cyclic structures is indeed somewhat tricky in a pure functional language. The obvious representation (representing the link structure implicitly in an algebraic data type) doesn't work, because you can't obvserve cycles. I recommend checking out two pieces of work in this area: 1. Martin Erwig's work on inductive representations of graphs: Inductive Graphs and Functional Graph Algorithms, Martin Erwig Journal of Functional Programming Vol. 11, No. 5, 467-492, 2001 available from: http://cs.oregonstate.edu/~erwig/papers/abstracts.html#JFP01 2. Koen Claessen and David Sands' work on observable sharing: Observable Sharing for Functional Circuit Description, Koen Claessen, David Sands, published at ASIAN '99, in Phuket, Thailand available from: http://www.math.chalmers.se/~koen/Papers/obs-shar.ps Hope that helps, -Antony -- Antony Courtney Grad. Student, Dept. of Computer Science, Yale University antony@apocalypse.org http://www.apocalypse.org/pub/u/antony