https://learn.microsoft.com/en-us/openspecs/office_file_formats/ms-xls/970af5a7-a51d-4a62-9255-01456033ebd6
[MS-XLS]: BuiltInFnGroupCount | Microsoft Learn
The BuiltInFnGroupCount record specifies the beginning of a collection of records as defined by the Globals Substream
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