Of these the most valuable is probably that of Guerra "Pontificarium Constitutionem in Bullario Magno contentarum Epitome" (4 vols., Venice, 1772), which possesses a very complete and useful index.
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In 1873 the Reverend Joseph Stevenson was sent to Rome for a similar purpose and transcripts made by him during four years' residence may be consulted at the Record Office, London.
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Some further help has been provided by Hampe, regarding the papal letters to Charlemagne and to Louis the Pious, and by Herth-Gerenth for Sergius II.