compute the necessary separation between two nodes
first
and second
are undefined
to indicate the separation from the
extents of the layout (0, or height). Both will never be undefined
.
sep - the minimum separation between the two nodes, regardless of which one is on top
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A separation function that indicates how far apart nodes should be the layering / height assignment.
Remarks
upper and lower are historic, since arbitrary graphs are handled, there is no longer a notion of upper or lower and separation should return the correct separation independent of nodes relations in the graph.