added two difficult pcp problems

pcp/pcp.hs

@@ -128,6 +128,9 @@ ba = LCStr $ \a b x -> b (a x)
 bb :: LCStr
 bb = LCStr $ \a b x -> b (b x)
 
+baa :: LCStr
+baa = LCStr $ \a b x -> b (a (a x))
+
 bba :: LCStr
 bba = LCStr $ \a b x -> b (b (a x))
 
@@ -244,6 +247,14 @@ problem_2 = cons (pair a abbb) (cons (pair bb b) nil)
 problem_3 :: ListStrPairs -- undecidable
 problem_3 = cons (pair bba b) (cons (pair b ab) (cons (pair a bba) nil))
 
+-- [(baa, b), (a, baa), (b, a)]
+problem_4 :: ListStrPairs -- has a solution of length 74
+problem_4 = cons (pair baa b) (cons (pair a baa) (cons (pair b a) nil))
+
+-- [(ab, a), (ab, bba), (a, baa), (baa, ba)]
+problem_5 :: ListStrPairs -- has a solution of length 76
+problem_5 = cons (pair ab a) (cons (pair ab bba) (cons (pair a baa) (cons (pair baa ba) nil)))
+
 -- PARSE LC ENCODINGS TO STRINGS
 
 -- string encoding to String