def sitespeptidesproteins(df, site_localization_probability=0.75):
    """
    Generate summary count of modified sites, peptides and proteins in a processed dataset ``DataFrame``.

    Returns the number of sites, peptides and proteins as calculated as follows:

    - `sites` (>0.75; or specified site localization probability) count of all sites > threshold
    - `peptides` the set of `Sequence windows` in the dataset (unique peptides)
    - `proteins` the set of unique leading proteins in the dataset

    :param df: Pandas ``DataFrame`` of processed data
    :param site_localization_probability: ``float`` site localization probability threshold (for sites calculation)
    :return: ``tuple`` of ``int``, containing sites, peptides, proteins
    """

    sites = filters.filter_localization_probability(df, site_localization_probability)['Sequence window']
    peptides = set(df['Sequence window'])
    proteins = set([str(p).split(';')[0] for p in df['Proteins']])
    return len(sites), len(peptides), len(proteins)