static fromJSON (json) {
      return new RangeNode(json.start, json.end, json.step)
    }
Instantiate an RangeNode from its JSON representation
@param {Object} json
    An object structured like
    `{"mathjs": "RangeNode", "start": ..., "end": ..., "step": ...}`,
    where mathjs is optional
@returns {RangeNode}