Add primegen example

Signed-off-by: Clifford Wolf <clifford@clifford.at>

docs/examples/puzzles/.gitignore

@@ -1 +1,2 @@
 /wolf_goat_cabbage
+/primegen

docs/examples/puzzles/primegen.sby

@@ -0,0 +1,13 @@
+[options]
+mode cover
+depth 1
+
+[engines]
+smtbmc --dumpsmt2 --stbv z3
+
+[script]
+read_verilog -formal primegen.v
+prep -top primegen
+
+[files]
+primegen.v

docs/examples/puzzles/primegen.v

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+module primegen;
+	wire [31:0] prime = $anyconst;
+	wire [15:0] factor = $allconst;
+
+	always @* begin
+		if (1 < factor && factor < prime)
+			assume((prime % factor) != 0);
+		assume(prime > 1000000000);
+		cover(1);
+	end
+endmodule