How a union or a difference goes through its shapes. oneAfterAnother takes one shape per step:
right for any shapes, and the edge and face numbering results have always had. inGroups puts shapes
whose boxes do not meet into one step: faster, numbered differently. allAtOnce takes every shape in
one step: fast, but wrong without an error where shapes touch each other at a single point.
How a union or a difference goes through its shapes. oneAfterAnother takes one shape per step: right for any shapes, and the edge and face numbering results have always had. inGroups puts shapes whose boxes do not meet into one step: faster, numbered differently. allAtOnce takes every shape in one step: fast, but wrong without an error where shapes touch each other at a single point.