def clusters_to_pixel_set(clusters):
    """
    Converts a list of clusters to a set of pixels.

    This function has no callers and is usually used as a one-liner.

    Parameters
    ----------
    clusters : list of list of tuple
        The outer list is a list of clusters. Each cluster is a list of (i, j)
        tuples marking the position of significant points which belong to that
        cluster.

    Returns
    -------
    set of tuple
        Each tuple is of the form (i, j) and marks the position of a significant
        point in the clustering.
    """
    return set().union(*clusters)