def feature_collection_polygons(featureCollection):
    """Turn a feature collection
    into an iterator over polygons.

    Given a featureCollection of the kind loaded from the json
    input, unpack it to an iterator which produces a series of
    (tzname, polygon) pairs, one for every polygon in the
    featureCollection.  Here tzname is a string and polygon is a
    list of floats.

    """
    for feature in featureCollection['features']:
        tzname = feature['properties']['TZID']
        if feature['geometry']['type'] == 'Polygon':
            exterior = feature['geometry']['coordinates'][0]
            interior = feature['geometry']['coordinates'][1:]
            yield tzname, (exterior, interior)
