Synopsis Arrow for source of an edge.
Function FProperty fromArrow(Figure f)
Description Add an arrow to the source of a graph edge.
The arrow is an arbitrary figure
An arrow to the destination of an edge is defined with toArrow. 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 destination of an edge is defined with toArrow. 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", fromArrow(box(size(20)))),
edge("A", "C", fromArrow(box(size(20))))
];
render(graph(nodes, edges, gap(50), std(size(100))));