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Doug Currie writes: > I agree that a circular-list should not be considered a `list' for the > purpose of the specification of safety preconditions. But, if it is > considered a `pair' then I don't think the precondition for LAST-PAIR is > sufficient. Yikes -- you're right. How about this? --------------------------------------------------------------------------- LAST-PAIR The argument is a list or an improper list. --------------------------------------------------------------------------- Whether this is an improvement seems to depend on how you read the definition of an improper list in R5RS: # A chain of pairs not ending in the empty list is called an improper # list. Can a chain contain a cycle? Maybe what we want to say is that the argument to LAST-PAIR must be a non-cyclic chain of pairs. -- ====== John David Stone - Lecturer in Computer Science and Philosophy ===== ============== Manager of the Mathematics Local-Area Network ============== ============== Grinnell College - Grinnell, Iowa 50112 - USA ============== ========== stone@xxxxxxxxxxxxx - http://www.math.grin.edu/~stone/ =========