title: How to reference to a response object with swagger?
body: <p>I can do this:</p>
  
  <pre><code>parameters:
    avatarSizeParam:
      name: size
      in: query
      description: Size of avatar.
      enum: [32, 64]
      required: false
      type: integer
      format: int32
  
  paths:
    /my/path/avatar:
      get:
        parameters:
          - $ref: '#/parameters/avatarSizeParam'
  </code></pre>
  
  <p>Good. Swagger defines a <code>parameters</code> key where you can define <code>Parameter Objects</code> to be
  reused. It also defines a <code>responses</code> key, where you can define <code>Response Objects</code> like
  so:</p>
  
  <pre><code>responses:
    notFoundResponse:
      description: Entity not found.
      schema:
        $ref: '#/definitions/schema404'
  </code></pre>
  
  <p>So I assumed I could expand my previous path definition to the following</p>
  
  <pre><code>paths:
    /my/path/avatar:
      get:
        parameters:
          - $ref: '#/parameters/avatarSizeParam'
        responses:
          - $ref: '#/responses/notFound'
  </code></pre>
  
  <p>This doesn't seem to work however. I went back to <a href="https://github.com/swagger-api/swagger-spec/blob/master/versions/2.0.md#operationObject">the spec</a> for an <code>Operations Object</code> and noticed that <code>parameters</code> can be a Reference Object, but <code>responses</code> cannot. </p>
  
  <p><img src="https://i.stack.imgur.com/v4ftM.png" alt="Operation Object definiton (partial)"></p>
  
  <p>What is the point of allowing a Responses Definitions Object (<code>responses</code> on the top-most level) if
  you can't reference the items there? Is there a way to do so?</p>