The final sheet is deliberately broken in two ways, and the engine handles both without flinching. The division A1/0 produces #DIV/0!, and because errors propagate, the cell that reads it (C1) becomes #DIV/0! too. The pair D1 and E1 reference each other, forming a cycle with no valid order; Kahn’s algorithm leaves them unplaced and recalculation flags both #CIRC!. Throughout, the healthy cell A1 keeps its value, and - the crucial part - Recalculate returns. It never chases the cycle in an infinite loop.

That is the whole promise kept. From A1 addressing you built a formula language, an evaluator, and a starter function library; then the real machinery of a spreadsheet - a dependency graph, a topological recalculation order, incremental updates that touch only what changed, and error handling that detects cycles and spreads failures honestly. It is a genuinely working calculation engine, the same design that sits underneath VisiCalc, Lotus 1-2-3, and Excel - the exact core those products extend with a grid, a file format, and hundreds more functions. That core is yours.