'Remove empty space after all non-zero elements.'

def prune(self):

    (R, C) = self.blocksize
    (M, N) = self.shape
    if (len(self.indptr) != ((M // R) + 1)):
        raise ValueError('index    pointer    has    invalid    length')
    bnnz = self.indptr[(-1)]
    if (len(self.indices) < bnnz):
        raise ValueError('indices    array    has    too    few    elements')
    if (len(self.data) < bnnz):
        raise ValueError('data    array    has    too    few    elements')
    self.data = self.data[:bnnz]
    self.indices = self.indices[:bnnz]
