elow, string based, and key based apis above
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VersionedFiles
  Provides .texts, .revisions etc
  This adapts the N-tuple keys to physical knit records which only have a
  single string identifier (for historical reasons), which in older formats
  was always the revision_id, and in the mapped code for packs is always
  the last element of key tuples.
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GraphIndex
  A separate GraphIndex is used for each of the
  texts/inventories/revisions/signatures contained within each individual
  pack file. The GraphIndex layer works in N-tuples and is unaware of any
  semantic value.
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