% problem-set/puzzles/truth.lies/oona.in % created : 08/08/88 % revised : 08/17/89 % description : % % a problem from Smullyan, Forever Undecided, p.25 % " In another curious incident, when the husband arrived on an island % looking for Oona, he met 5 natives A,B,C,D,E who all guessed his % purpose and grinned at meeting him. They said: % A: Oona is on this island. B: Oona is not on this island. C: Oona % was here yesterday. D: Oona is not here today, and she was not here % yesterday. E: Either D is a knave or C is a knight. % The logician thought for a while, but could get nowhere. `Won't one % of you please make another statement?' the logician pleaded. At this % point A said: Either E is a knave or C is a knight. % Is Oona on the island?" % representation : % % Atoms a,b,c,d,e stand for the native's being knights (positive) or % knaves (negative); statement implications have been translated. % Assume Oona is not on the island, i.e. assume b. set(ur_res). set(unit_deletion). list(axioms). -a | -b. -a | -d. -b | -a. b | a. -b | c | d. -c | -d. -d | -a. -d | b. -d | -c. -e | -d | c. e | d. e | -c. d | a | c. d | -b | c. -a | -e | c. a | e. a | -c. end_of_list. list(sos). b. end_of_list.