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Previously CellValue had three variants: Number, Text, and Empty. The Empty variant acted as a null sentinel, but the compiler could not distinguish between "this is a real value" and "this might be empty". Code that received a CellValue could use it without checking for Empty, because there was no type-level enforcement. Now CellValue has only Number and Text. The absence of a value is represented as None at every API boundary: DataStore::get() → Option<&CellValue> (was &CellValue / Empty) Model::get_cell() → Option<&CellValue> (was &CellValue / Empty) Model::evaluate() → Option<CellValue> (was CellValue::Empty) eval_formula() → Option<CellValue> (was CellValue::Empty) Model gains clear_cell() for explicit key removal; ClearCell dispatch calls it instead of set_cell(key, CellValue::Empty). The compiler now forces every caller of evaluate/get_cell to handle the None case explicitly — accidental use of an empty value as if it were real is caught at compile time rather than silently computing wrong results. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>