v(...)
u(...)
d(...)
r(...)v
  and strictly positive integers for u,d and r)v returns an object of  class "vertexSet".
Function u returns an object of  class "undirectedEdge".
Both functions d and r return an object of  class "directedEdge".
d builds a directed edge from tail to head,
function r builds a directed edge from head to tail.vertexSet-class,
undirectedEdge-class and
directedEdge-class.