An arc is inbound to a vertex if that vertex is the 'head' of the arc.
Returns a generator that yields 2-tuple (array) where the first two values
represent the vertex pair. Vertex order is guaranteed: the first vertex
is the tail, and the second vertex is the head.
If the graph has additional edge data (e.g., weight), additional elements
are appended to the edge array as needed. (See implementation-specific
documentation for more detail).
См. также:
Digraph::arcsFrom()
См. также:
Digraph::predecessorsOf()
См. также:
Graph::incidentTo() Note that, in set terms,
g.incidentTo(x) == g.arcsFrom(x) ∪ g.arcsTo(x)