/// <inheritdoc />
        public bool ContainsVertex(TVertex vertex)
        {
            if (vertex == null)
            {
                throw new ArgumentNullException(nameof(vertex));
            }

            return(_adjacentEdges.ContainsKey(vertex));
        }
 public bool ContainsVertex(TVertex v)
 {
     return(vertexOutEdges.ContainsKey(v));
 }