def flatten_documents_to_sentence_strings(docs):
    """Flattens the given documents in nested form to a string representation where each sentence is a new document (useful for sentence-wise cooccurrence measuring)

    [ #docs
        [ #document1
            ['word1','word2','word3'], #sentence1
            ['word1','word2','word3'], #sentence2
        ],
        [ #document2
            ['word1','word2','word3'], #sentence1
            ['word1','word2','word3'], #sentence2
        ]
    ]

    becomes

    [ #docs
        's1_word1 s1_word2 s1_word3', #document1_sentence1
        's2_word1 s2_word2 s2_word3', #document1_sentence2
        's1_word1 s1_word2 s1_word3', #document2_sentence1
         s2_word1 s2_word2 s2_word3', #document2_sentence2
    ]
    """
    strsents = []
    for doc in docs:
        strsents.extend([' '.join(sent) for sent in doc])
    return strsents