A selected-edge-is-a-source-not-a-caption scan holds the period fixed and varies the selected edge index across three rows, checking that each cited edge time t_n = n * T is bound as a source for later timing checks row by row. The arithmetic is exact only for the stated periods, edge indices, edge times, source citations, and rows in the ideal periodic-clock model. Real clocks add jitter, duty-cycle distortion, skew, finite rise time, threshold uncertainty, probe/calibration limits, finite precision, and measurement uncertainty.
The chosen capture edge must be cited by downstream checks. 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 selected edge is a source value
The capture source is edge 1 at 8 seconds.
tedge=8s
Different selected edges expose different source times
The period stays fixed; only the selected edge index changes.
n123tn8s16s24ssourceedgeedgeedge
Later timing checks cite this same edge
A later setup or capture check cannot silently move edge 1 to another time.