/// <summary>
        /// Delete the vertex, along with all its adjacenct edges.
        /// </summary>
        private void Vertex_KeyPress_Delete(object sender, KeyPressEventArgs e)
        {
            int vertexNameIndex = (sender as Vertex).GetNumberIndex();

            vertexNameUsed[vertexNameIndex] = false;
            if (vertexNameIndex < defaultVertexNameCounter - 'A')
            {
                defaultVertexNameCounter = Convert.ToChar(vertexNameIndex + 'A');
            }
            vertices.Remove(sender as Vertex);
            foreach (Vertex v in vertices)
            {
                if (v.ContainsEdge(sender as Vertex))
                {
                    v.RemoveEdge(sender as Vertex);
                }
            }
            mapMatrix.RemoveVertex(vertexNameIndex);
            mapList.RemoveVertex(vertexNameIndex);
            ResetBoard();
            vStart  = null;
            vFinish = null;
        }