'SAS'-Style 'PROC FORMAT' for R // Provides 'SAS' 'PROC FORMAT'-like functionality for creating and applying value formats in R. Supports discrete and range-based mapping of values to labels, reverse formatting (invalue), date/time/datetime formatting with built-in 'SAS' format names, multi-label formats, expression labels evaluated at apply-time, case-insensitive matching, import/export of format definitions, and proper handling of missing values (NA, NULL, NaN).