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A latent index-based select condition is not a BFS-level function. If an indexed field is passed as a sort criterion in a filing system SELECT call, SI.MFS traps the call. Other MFSs that follow SI.MFS, and the actual BFS, never receive the SELECT call. SI.MFS returns a select mode of 2 in the RECORD argument (causing @LIST.ACTIVE to be set to 2), indicating an index-based select condition.
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