Hello again, since Oleg presented an approach to the sharing problem that works on acyclic graphs, I may as well mention an alternative, pure, standard Haskell solution which is to express the fork points in the circuit (or expression), i.e. the points at which an expression is duplicated. You need to introduce a fork primitive: fork :: Bit -> (Bit, Bit) or fork :: Exp -> (Exp, Exp) This fits quite naturally in the context of circuit description, and I called it "expressible sharing". Under some mild restrictions, it even works on cyclic graphs. In particular, it works nicely for regular circuits. The "tricky" example I mentioned earlier would be written: tricky 0 = constant 0 tricky d = add e0 e1 where (e0, e1) = fork (tricky (d-1)) However, in the end I admitted defeat and decided I preferred observable sharing! Matt.