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