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I extruded a rectangle (yellow) and the extrusion passed through an existing solid (green).
What I thought I would get is an interference pattern as a visual clue but the action did a subtract on the existing solid. All 4 pieces of the angle iron base were identical before the extrusion. Now two have cuts in their vertical sides.
I checked the EXTRUDE docs and saw no reference (that I understood) to change the behavior to not subtract, but to interfere. Is there a setting?