An improved method and apparatus for quickly and efficiently updating the original source volume and original target volumes after the original source volume has become temporarily unavailable. Cells bounding a region defined along a portion of a stored logically-defined grid are identified and each cell is added into an element of a level set array block in order of traversal through the region along the boundary. When the injector 8 is determined to be in malfunction, an operation of temporarily sucking back the urea water 12 in the passage is interposed before the pump 13 is stopped; the pump 13 is restarted after lapse of a predetermined time to re-determine whether the injector 8 is in malfunction or not. The pressure-activated switch is capable of detecting an internal pressure in the bottle. The fastener further includes a plurality of spaced apart lugs encircling the central pilot portion and axially extending from at least one of the end face and the groove.