childrenOf method
Implementation
Map<V, Vertex<V>> childrenOf(Vertex<V> vertex) {
if (!vertices.containsValue(vertex)) return {};
final children = uniqueEdges.keys
.where((edgeId) => edgeId.$1 == vertex.id)
.map((edgeId) => edgeId.$2)
.where((id) => vertices.containsKey(id))
.map((id) => MapEntry(id, vertices[id]!));
return Map.fromEntries(children);
//.where((entry) => !a.containsKey(entry.key)));
}