Introduction | Motivation | Definitions | Example | User-made Index In Lookup | Lookup With User-made Index | Zero Value Present | Empty (NA) Value Present | Row Number Present | Lookup Without User-made Index | Index <span style='color: red;'>Must</span> Be Absent In Lookup | Index <span style='color: red;'>May</span> Be Absent In Lookup | Functions Intended For User | Internal Workflow | If The Index Is Found Absent | Internal Index For Duplicated Values | No Internal Index For Missing Values | Index For Unique Values | If The Index Is Found Present | Screen Output Example | Example Datasets | Data And Associated Lookup Files With Index | Data And Associated Lookup Files Without Index | Other Associated Lookup Files | Session Info