spans = (graph1, graph2) => {
    let span;
    if (graph1.vertices.size === graph2.vertices.size) {
      span = true;
      graph1.vertices.forEach(v => {
        if (!graph2.vertices.contains(v)) {
          span = false;
        }
      });
    } else {
      span = false;
    }
    return span;
  }
Test whether two graphs share the same vertex set.

@param {Graph} graph1
@param {Graph} graph2
@return {boolean}