Predecessor patch

[alexC-nonsense4k]

The coupling issue arises in the viterbi_search function due to its reliance on the get_predecessor function. One concern with this coupling is within the get_predecessor function, where a key is retrieved from a dictionary using the get operation. The key might not always exist in the dictionary, but I've implemented a check in get_predecessor to ensure the program won't crash if attempting to get a non-existent key. In such cases, get_predecessor will return an empty dictionary.

I've also added comments in the viterbi_search function to emphasize that get_predecessor should be used separately. Additionally, it's crucial to verify whether the returned predecessor is empty after calling get_predecessor.