code: function link(edge) {
    var es = edge.s.euler
    var et = edge.t.euler
    var euler = new Array(edge.level+1)
    for(var i=0; i<euler.length; ++i) {
      if(es.length <= i) {
        es.push(createEulerVertex(edge.s))
      }
      if(et.length <= i) {
        et.push(createEulerVertex(edge.t))
      }
      euler[i] = es[i].link(et[i], edge)
    }
    edge.euler = euler
  }
docs: Add an edge to all spanning forests with level <= edge.level