adding xor circuit and tutorial family tree

README.md

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+# Logic Problems
+
+Working through some logic programming problems using Prolog. Tutorial problems from post-grad AI and AI programming module with some other random problems.

familytree.pl

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+% tree structure
+parent(pam,bob).
+parent(tom,bob).
+parent(tom,liz).
+parent(bob,ann).
+parent(bob,pat).
+parent(pat,jim).
+
+predecessor(X, Z) :- parent(X, Z).
+predecessor(X, Z) :- parent(X, Y), predecessor(Y, Z).
+
+offspring(Y, X) :- parent(X, Y).

xor.pl

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+
+% implement logical AND truth table
+% and(IN1, IN2, OUT)
+and(0,0,0).
+and(0,1,0).
+and(1,0,0).
+and(1,1,1).
+
+% implement logical OR truth table
+% or(IN1, IN2, OUT)
+or(0,0,0).
+or(0,1,1).
+or(1,0,1).
+or(1,1,1).
+
+% implement logical inverter
+% inv(IN, OUT)
+inv(0,1).
+inv(1,0).
+
+% implement XOR using rule
+xor(IN1,IN2,OUT) :- inv(IN1, N1),
+                    inv(IN2, N2),
+                    and(IN1, N2, N3),
+                    and(IN2, N1, N4),
+                    or(N3, N4, OUT).