Align objects `relative` to an outermost edge of a set of objects.
At `p0`: (1) A straignt line starts. (2) An angle has its arrow head here. Line and angle define one of the directions left, right, top, or bottom. (3) A selector rectangle defines the set of objects to align.
Align a set of objects with their right edges to the right edge of the set.
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Align a set of objects with their left edges to the left edge of the set.
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Align a set of objects with their bottom edges to the bottom edge of the set.
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Align a set of objects with their top edges to the top edge of the set.
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