code: function slack(graph, u, v, minLen) {
    return Math.abs(graph.node(u).rank - graph.node(v).rank) - minLen;
  }
docs: /*
  A helper to calculate the slack between two nodes (`u` and `v`) given a
  `minLen` constraint. The slack represents how much the distance between `u`
  and `v` could shrink while maintaining the `minLen` constraint. If the value
  is negative then the constraint is currently violated.
  
  This function requires that `u` and `v` are in `graph` and they both have a
  `rank` attribute.