roots property
override
Each node that is exposed as a root node of the walkable collection.
A root node is implementation defined, but is typically a node that has one or more successors, but not necessarily. For example, a graph may have multiple root nodes, or choose to expose every node in the graph as a root node, while a tree may have only one root node.
A collection that has no root nodes is considered empty.
Implementation
@override
Iterable<Pos> get roots {
return _GridRootsIterable(this);
}