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