code: public void removeEdge(V from, V to) {
          if (!containsVertex(from)) {
              throw new IllegalArgumentException("Nonexistent vertex " + from);
          }
  
          if (!containsVertex(to)) {
              throw new IllegalArgumentException("Nonexistent vertex " + to);
          }
  
          neighbors.get(from).remove(to);
      }
docs: Remove an edge from the graph. Nothing happens if no such edge.
  @throws {@link IllegalArgumentException} if either vertex doesn't exist.