title: Subpaths in Swagger YAML declaration
body: <p>I am trying to create a REST service by describing it in Swagger YAML.</p>
  
  <p>The service has three paths:</p>
  
  <ul>
  <li>/versions</li>
  <li>/partners/{partnerId}/users/{userId}/sessions</li>
  <li>/partners/{partnerId}/books/{bookId}/</li>
  </ul>
  
  <p>My current YAML file to describe these paths looks like this:</p>
  
  <pre><code>swagger: '2.0'
  info:
    version: '0.0.1'
    title: Test API
  host: api.test.com
  basePath: /
  schemes:
    - https
  consumes:
    - application/json
  produces:
    - application/json
  paths:
    /versions:
      post:
        responses:
          '201':
            description: Returns all versions.
          default:
            description: unexpected error
    /partners/{partnerId}/users/{userId}/sessions:
      parameters:
        - name: partnerId
          in: path
          type: integer
        - name: userId
          in: path
          type: string
      post:
        responses:
          '201':
            description: Returns a UserSession object with info about the user session.
          default:
            description: unexpected error
    /partners/{partnerId}/books/{bookId}/:
      parameters:
        - name: partnerId
          in: path
          type: integer
        - name: bookId
          in: path
          type: string
      get:
        responses:
          '200':
            description: Gets a book.
          default:
            description: unexpected error
  </code></pre>
  
  <p>In this YAML file the parameter "partnerId" is declared twice.</p>
  
  <p>Is there a way to make "subpaths" such that I don't have to declare the <code>/partners/{partnerId}</code> part of the path twice?</p>