C4: constructed primitive extensions and the joint stop

The constructed contacts belong to the allowed partial-diagonal form whenever their spectator product is defined. For $y=v$, the spectator can still connect $v$ to $x$. The contact is not restricted to $x=y=v$.

The oriented $H$ parallel-line product is already defined by the inherited product calculus. Subtracting it from an extended time-ordered primitive therefore constructs the corresponding primitive difference. Since the difference has its prescribed values off $r=0$, any added contact changes only that collision.