Finds the next edge with the minimum slack.
function findMinSlack() {
    var result,
        eSlack = Number.POSITIVE_INFINITY;
    minLen.forEach(function(mle /* minLen entry */) {
      if (remaining.has(mle.u) !== remaining.has(mle.v)) {
        var mleSlack = rankUtil.slack(g, mle.u, mle.v, mle.len);
        if (mleSlack < eSlack) {
          if (!remaining.has(mle.u)) {
            result = {
              treeNode: mle.u,
              graphNode: mle.v,
              len: mle.len,
              reversed: false,
              weight: mle.weight
            };
          } else {
            result = {
              treeNode: mle.v,
              graphNode: mle.u,
              len: -mle.len,
              reversed: true,
              weight: mle.weight
            };
          }
          eSlack = mleSlack;
        }
      }
    });

    return result;
  }