Edge getFirstIncomingEdge(Vertex v) {
		//return firstIncomingEdge[ index( v ) ];
		try {
			return firstIncomingEdge[keyIndices.get(v.key())];
		}
		catch (NoValueException exc) {
			return null;
		}
	}
Get the first incoming edge in the internal structures for this 
VertexSet for the parent DirectedGraph.
Ghidra/Framework/Generic/src/main/java/ghidra/util/graph/VertexSet.java