/*
Return an edge from the tree with a negative cut value, or null if there
is none.
function leaveEdge(tree) {
  var edges = tree.edges();
  for (var n in edges) {
    var e = edges[n];
    var treeValue = tree.edge(e);
    if (treeValue.cutValue < 0) {
      return e;
    }
  }
  return null;
}