(make-graph) -> graph
(make-node label) -> node
(node-label node) -> label
(add-node graph node) -> graph
(add-edge graph from-node to-node) -> edge
(edge-from edge) -> from-node
(edge-to edge) -> to-node
(graph-edges graph) -> {edge}
(graph-nodes graph) -> {node}
(graph-edges-from graph node) -> {edge}