Restrict response to only fields defined in RAML

Hi, this is a newbie question. if I define 3 optional fields (say a, b, c) then of course it is OK to get a response with any of them missing. But I also noticed I could receive new fields which I did not define.
In above example, how to make sure apart from (a, b, c) I don’t get any (d,e,f,g…)?

additionalProperties: false (see the Additional Properties section of the RAML 1.0 spec for more info)