a reference plane setting section for setting a virtual reference plane as a reference defining {HEAD ~ a geometric placement} of {TAIL ~ an arc} welding torch relative to a workpiece during a welding operation, based on said plurality of position and orientation data obtained in said data obtaining section 
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a reference plane setting section for setting a virtual reference plane as a reference defining { HEAD ~ a geometric placement } of { TAIL ~ an arc } welding torch relative to a workpiece during a welding operation , based on said plurality of position and orientation data obtained in said data obtaining section
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