title: Pre-Requisite for Graphs with Unique Topological Sort
body: <p>Let's assume that a graph in question is a DAG (directed acyclic graph). </p>
  
  <p><strong>Question</strong>: can I conclude that such graph will have a unique topological sort if, and only if, only one of its vertices has no incoming edges?</p>
  
  <p>In other words, is having only one vertex with no incoming edges necessary (but not sufficient) to generate a unique topological sort?</p>