code: function enterEdge(graph, tree, e) {
    var source = tree.source(e);
    var target = tree.target(e);
    var lower = tree.node(target).lim < tree.node(source).lim ? target : source;
  
    // Is the tree edge aligned with the graph edge?
    var aligned = !tree.edge(e).reversed;
  
    var minSlack = Number.POSITIVE_INFINITY;
    var minSlackEdge;
    if (aligned) {
      graph.eachEdge(function(id, u, v, value) {
        if (id !== e && inSubtree(tree, u, lower) && !inSubtree(tree, v, lower)) {
          var slack = rankUtil.slack(graph, u, v, value.minLen);
          if (slack < minSlack) {
            minSlack = slack;
            minSlackEdge = id;
          }
        }
      });
    } else {
      graph.eachEdge(function(id, u, v, value) {
        if (id !== e && !inSubtree(tree, u, lower) && inSubtree(tree, v, lower)) {
          var slack = rankUtil.slack(graph, u, v, value.minLen);
          if (slack < minSlack) {
            minSlack = slack;
            minSlackEdge = id;
          }
        }
      });
    }
  
    if (minSlackEdge === undefined) {
      var outside = [];
      var inside = [];
      graph.eachNode(function(id) {
        if (!inSubtree(tree, id, lower)) {
          outside.push(id);
        } else {
          inside.push(id);
        }
      });
      throw new Error('No edge found from outside of tree to inside');
    }
  
    return minSlackEdge;
  }
docs: /*
  The edge e should be an edge in the tree, with an underlying edge
  in the graph, with a negative cut value.  Of the two nodes incident
  on the edge, take the lower one.  enterEdge returns an edge with
  minimum slack going from outside of that node's subtree to inside
  of that node's subtree.