' On (the) top of the mountain/building/bookcase/Christmas tree' -- both seem to apply to equally well to vertical physical objects. ' On top of everything else/the rest of my problems/all his other idiosyncrasies'-- only this form seems to work with metaphorical tops. ' On the top of the paper/ the label' -- these seem to take only 'the top', when we mean the upper edge ('on top of' the paper would be anywhere on the upper face).
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