commonPart :: String -> String -> Int -> Int
commonPart (m1:m1s) (m2:m2s) counter
  | m1 == m2 = 1 + commonPart m1s m2s counter
  | otherwise = counter
commonPart :: String -> String -> Int -> Int
commonPart (m1:m1s) (m2:m2s) counter
  | m1 == m2 = 1 + commonPart m1s m2s counter
  | otherwise = counter
commonPart (m1:m1s) (m2:m2s) counter
  | m1 == m2 = 1 + commonPart m1s m2s counter
  | otherwise = counter