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Most objects that occupy a two-dimensional area (polygons, circle interiors, arc interiors, pictures, and iterations and loci of these objects) appear in a layered order when they overlap.
To change the layer of a selected object, choose Bring to Front or Send to Back from the object’s Context menu.
Layered objects are also translucent: you can set their opacity to allow similar objects below them to show through. To change the opacity of a layered object, select it and choose Edit | Properties | Opacity.