Add wolf_goat_cabbage.v puzzle solver

docs/examples/puzzles/wolf_goat_cabbage.v

@@ -0,0 +1,38 @@
+// A man needs to cross a river with a wolf, a goat and a cabbage. His boat is only large
+// enough to carry himself and one of his three possessions, so he must transport these
+// items one at a time. However, if he leaves the wolf and the goat together unattended,
+// then the wolf will eat the goat; similarly, if he leaves the goat and the cabbage together
+// unattended, then the goat will eat the cabbage. How can the man get across safely with
+// his three items?
+
+module wolf_goat_cabbage (input clk, input w, g, c);
+	// everyone starts at the 1st river bank
+	reg bank_w = 0; // wolf
+	reg bank_g = 0; // goat
+	reg bank_c = 0; // cabbage
+	reg bank_m = 0; // man
+
+	always @(posedge clk) begin
+		// maximum one of the control signals must be high
+		assume(w+g+c <= 1);
+
+		// we want wolf, goat, and cabbage on the 2nd river bank
+		cover(bank_w && bank_g && bank_c);
+
+		// don't leave wolf and goat unattended
+		if (bank_w != bank_m) begin
+			assume(bank_w != bank_g);
+		end
+
+		// don't leave goat and cabbage unattended
+		if (bank_g != bank_m) begin
+			assume(bank_g != bank_c);
+		end
+
+		// man travels and takes the selected item with him
+		if (w && (bank_w == bank_m)) bank_w <= !bank_m;
+		if (g && (bank_g == bank_m)) bank_g <= !bank_m;
+		if (c && (bank_c == bank_m)) bank_c <= !bank_m;
+		bank_m <= !bank_m;
+	end
+endmodule