message: sets arc_top to target Z if higher than arc_top; sets previous_instrument to ensure correct retract behavior
diff: @@ -628,6 +628,8 @@ class Robot(object):
  # Z arc height will be used for the new pipette
  self._prev_container = None
  + self._previous_instrument = instrument
  +
  if strategy == 'arc':
  arc_coords = self._create_arc(target, placeable)
  for coord in arc_coords:
  @@ -669,6 +671,7 @@ class Robot(object):
  # TODO (andy): there is no check here for if this height will hit
  # the limit switches, so if a tall labware is used, we risk collision
  arc_top += self.arc_height
  + arc_top = max(arc_top, destination[2])
  strategy = [
  {'z': arc_top},
  