def new_material(c1, c2, idx=0):
    """
    Return new material in a list. 
    'idx' is an optional parameter for cutting off references.
    """
    ctx1 = c1.view_contexts('sentence', as_strings=True)
    
    if idx == 0:
        ctx2 = c2.view_contexts('sentence', as_strings=True)
    else:
        ctx2 = c2.view_contexts('sentence', as_strings=True)[:idx]
    len2 = len(ctx2)

    new = []
    for i in xrange(len(ctx1)):
        if i < len2:
            if len(ctx1[i]) == 0: # empty tokens.
                pass
            else:
                ind = find_idx(i, c1, c2)
                if len(ind) == 0:
                    new.append(i)
    return new
dataset/ETHPy150Open inpho/vsm/vsm/extensions/editions.py/new_material/original