What is a tree?

Tree
A tree is a connected simple graph with no simple cycles.

In a tree, there is a unique simple path between any two of its vertices.

If we add an edge to a tree, it creates a cycle.

If we remove an edge from a tree, it becomes not connected.
graph G {
	layout=neato
	node[shape=point]
	4--1,2,3,5
	5--6,7,8
	8--9,10,11
}
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