The finite toy rule allows only parity-changing transitions. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The toy rule allows parity changes
The middle level is odd and the upper level is even, so the allowed bit is 1.
odd→even,allowed=1
The same rule rejects unchanged parity
The ground level and upper level are both even, so the allowed bit is 0.