This is a completely new implementation of Gull syzygy, and is not related to the previous syzygy patch.
Gull+syzygy showed improvement over straight non-syzygy Gull:
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Gull 5-man syzygy (4000 games) using 60"+0.4" 2-threads: Score: +673-565=2762 Winning fraction: 0.5135 Elo difference: +9.38304 LOS: 0.998928
Fathom is a standalone version of Ronald de Man's syzygy probing code. The original probing code depended on Stockfish functionality, and this must be replaced in order to integrate into a different engine. This is not so easy for an engine like Gull, which is different and does not have well-documented code.
Fathom aims to ease integration by making the probing code engine-independent. Basic integration is therefore reduced to three API calls: initialization, WDL-probe during search, and root-probe at the root.
As an example, the Gull Fathom patch is here: https://github.com/basil00/Gull/commit/ ... efdccaec0c
Although Fathom can be engine independent, some functionality can be replaced by the engine's code for better efficiency. This is optional, see the Gull patch above for an example.
Fathom can also be used for other projects, e.g. syzygy tablebase adjudication. This may be useful for GUIs. The Fathom repository also includes a stand-alone command-line tool for mapping FEN position strings to table-base results.