Dinner Split

Splitting a bill is arithmetic until it is not. Divide $100 three ways and the cents do not close: three shares of $33.33 sum to $99.99. Something has to absorb the difference, and the choice is a business rule, not a rounding mode.

Here the host absorbs it. Per-attendee :owes is rounded at the boundary, and the host's share takes the drift so that the invariant holds:

Σ(round attendee.owes) == round(grandTotal)

That is enforced by the property test roundoff-host-absorbs-drift rather than left as a comment. Arithmetic is double throughout, rounded once at the edge – never accumulated pre-rounded.