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import collections


def get_keys(d, keys=None, pointer=None, sep='.', prefix=''):
    if keys is None:
        keys = set()

    if pointer is None:
        pointer = []

    for key, value in d.items():
        keyptr = pointer + [str(key)]
        keys.add(prefix + sep.join(keyptr))
        if isinstance(value, collections.Mapping):
            keys |= get_keys(value, None, keyptr, sep, prefix)
    return keys

keys = (get_keys(i) for i in items)