def _update_freq_matrix(freq_matrix, indices, freq):
    """
    This updates the frequency matrix by subtracting the supplied frequency from
    the found reads.

    Parameters
    ----------

    freq_matrix : bioa.FrequencyMatrix
        The frequency matrix over the reads.

    indices : list
        List of indices where the chosen reads were in the original matrix.

    freq : float
        The frequency to deduct from other reads.

    Returns
    -------
    None
    """
    for cdx, rdx in enumerate(indices):
        read, val = freq_matrix[cdx][rdx]
        val = max([0, val - freq])
        freq_matrix[cdx][rdx] = read, val
    return
