until n s0 s1 means that eventually n s1, and up
until at least the period before s1 holds, s0
continuously holds.
Note: Both argument streams must have sufficient history to
dropn values from them.
True until another stream is true, within the time bounds specified.
until l u clk dist s0 s1 is true at time t iff there
exists a d, with l <= d <= u, such that
s1 is true at time (t + d), and for all times
t' with t <= t' < t + d, s0 is true
at those times.