[Graph] - DAG : An important observation

In a DAG, a mutually unreachable pair (s,t) can exist if and only if there exists at least one topological level with at least two nodes. 

Read the post at Notion : https://www.notion.so/DAG-Directed-Acyclic-Graph-1d8d32fa6ba880e7aea2d3eb59ad46e1