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