Two syndrome bits make four table rows, so double-error aliases cannot be new rows. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Two syndrome bits give four table rows
The table capacity is fixed by the two checked parity bits.
Nrows=4
Demand rows bracket the syndrome table capacity
Adding the zero-syndrome row fills the table. Treating a double error as new demand overfills it.
Ndemand345Nrows444m10−1gatepasspassfail
A double-error class aliases an existing row
The lookup helper shows the double-error syndrome is not a new independent table row.