How to Refer Key and Value in Key-Value pair in OPA Gatekeeper in Rego

Symptom:

    We start to use OPA gatekeeper for our kubernetes clusters. Refer https://github.com/open-policy-agent/gatekeeper for more details.
    When we code some policies for kubernetes using OPA (open policy agent) Rego ,we would like to reference "key" name and "value"  in nodeSelector key-value pair.  ie we have

nodeSelector:
    app: mytest

I would like to refererence "app" which is key and "test" which is value in our OPA gatekeeper policy .

Solution:

   The easy way to do it is

 myvalue := input.review.object.spec.nodeSelector[mykey]

The value of varible mykey will have  "app"
The value variable myvalue will have  "mytest"
And they are strings

To get "set" , we need to use special way to achieve it:
To get "set" for key : 

  provided := {mykey | input.review.object.spec.nodeSelector[mykey]}

To get set for value: 

  provided := {myvalue | myvalue := input.review.object.spec.nodeSelector[_]}