(byte string):
    A non-secret, reusable value that strengthens the randomness
    extraction step.
    Ideally, it is as long as the digest size of the chosen hash.
    If empty, a string of zeroes in used.
 hashmod (module):
    A cryptographic hash algorithm from :mod:`Cryptodome.Hash`.
    :mod:`Cryptodome.Hash.SHA512` is a good choice.
 num_keys (integer):
    The number of keys to derive. Every key is :data:`key_len` bytes long.
    The maximum cumulative length of all keys is
    255 times the digest size.
 context (byte string):
    Optional identifier describing what the keys are used for.

Return:
    A byte string or a tuple of byte strings.

.. _RFC5869: http://tools.ietf.org/html/rfc5869
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