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3.1.10 CRL objects

CRL objects have the following methods:

`add_revoked`(revoked)
  Add a Revoked object to the CRL, by value not reference.

`export`(cert, key[, type=FILETYPE_PEM][, days=100])
  Use cert and key to sign the CRL and return the CRL as a string. days is the number of days before the next CRL is due.

`get_revoked`()
  Return a tuple of Revoked objects, by value not reference.