def _look_before (index_sentence,context) :
    """Generate the look before context starting with the sentence index and looking no less than the first sentence"""

    context_pairs=[]
    for i in range(1,context+1) :
        s_index=index_sentence-i
        if  s_index>=0 :
            context_pairs.append(( s_index,index_sentence))
    return context_pairs