import collections


def _aggregate_histograms(histograms):
    """Aggregate multiple histograms into one.

    A single histogram just contains the number of times each row length
    occurred in the dataset.

    :arg list histograms: The histograms, as collections.Counter objects.
    :returns: A histogram
    :rtype: collections.Counter
    """
    aggregated = collections.Counter()
    for hist in histograms:
        aggregated.update(hist)
    return aggregated