setDefaultEdgeLabel(newDefault) {
    this._defaultEdgeLabelFn = newDefault;
    if (typeof newDefault !== 'function') {
      this._defaultEdgeLabelFn = () => newDefault;
    }

    return this;
  }
Sets the default edge label or factory function. This label will be
assigned as default label in case if no label was specified while setting
an edge or this function will be invoked each time when setting an edge
with no label specified and returned value * will be used as a label for edge.
Complexity: O(1).