Synopsis Add an error to the destination of an edge.
Function FProperty toArrow(Figure f)
Description Add an arrow to the destination of a graph edge.
The arrow is an arbitrary figure
An arrow to the source of an edge is defined with fromArrow. An edge may have both a fromArrow and a toArrow property.
f and its upper side will be used as the
point of the arrow. The figure is appropriately rotated depending on the orientation of the edge.
An arrow to the source of an edge is defined with fromArrow. An edge may have both a fromArrow and a toArrow property.
Examples 
nodes = [ box(id("A"), fillColor("red")),
box(id("B"), fillColor("yellow")),
box(id("C"), fillColor("blue"))];
edges = [ edge("A", "B", toArrow(box(size(20)))),
edge("A", "C", toArrow(box(size(20))))
];
render(graph(nodes, edges, gap(50), std(size(100))));