Task criterion: ntd-participation
When searching for a step, this criterion means pick the next step in
which the NTD is either the CMD or the RESP, and which is not crossed
off. If the first such step that is found has the NTD as the RESP and
has a time value that is later in the launch sequence than the current
countdown clock, the NTD will pause there and wait for time to pass.
(This is obviously not the only possible activity.)