Move a video or audio clip by delta_ms between its neighbours on its track: the clip touching its start gets longer or shorter at its end, the clip touching its end at its start, so the three-clip span and every other track stay put. A side with a gap instead lets the clip move into the gap and no further. Refuses, naming the fix: an unknown id, a scene (scenes reorder with reorder_scenes), text or a still image (move it with shift_clips), a locked track, or a delta that would leave a neighbour under 100ms or past its footage (the refusal gives the delta_ms range that fits).