JSON Activity Streams 2.0 - Action Handlers
draft-snell-activitystreams-actions-03

Abstract

This specification defines Action Handlers for use with the Activity Streams 2.0 format.

Author's Note

Note that this document is a work-in-progress draft specification that does not yet represent a "standard". It is the intention of this specification to propose a few new ideas and openly solicit feedback on their definition and use. While this document might eventually evolve into an RFC the ideas described herein have not yet been broadly implemented and have definitions that will evolve through successive iterations of this draft.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on July 30, 2014.

Copyright Notice

Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Overview
• 2. Action Handlers
• 3. HTTP Action Handler
• 4. Embed Action Handler
• 5. Intent Action Handler
• 6. Using "service" and "application" objects as action handlers
• 7. HTML Form Objects
• 8. Typed Payload Objects
• 9. URL Template Objects
• 10. Parameters Object
• 10.1. Parameter Object
• 10.2. Using UrlTemplate and TypedPayload objects as parameter descriptions
• 11. Authentication Object
• 12. Styles Object
• 13. Security Considerations
• 14. IANA Considerations
• 15. Normative References
• Appendix A. Using Action Handlers From Other Vocabularies
• A.1. Schema.org Actions Proposal
• A.2. Google's "Actions in the Inbox"
• A.3. Mixing Vocabularies
• A.4. Example Drawing From Multiple Vocabularies
• Authors' Addresses

1. Overview

The Activity Streams 2.0 [I-D.snell-activitystreams] specification introduces the notion of "actions" that can be associated with objects. Using the "actions" property described in Sections 3.6 and 3.6.1 of the Activity Streams 2.0 document, the producer of an object can declare a specific set of verbs appropriate for the object and map each of those to one or more objects ("action handlers") or resources capable of "carrying out" the verb. This document expands on that mechanism by defining and describing a core set of action handler object types.

2. Action Handlers

An action handler is an Activity Streams 2.0 object whose objectType and member properties instruct a consuming application how to carry out the verb the action handler has been associated with. For instance, given the following example:

  {
    "objectType": "note",
    "displayName": "Title of the note",
    "content": "This is a simple note.",
    "actions": {
      "share": {
        "objectType": "HttpActionHandler",
        "url": "http://example.org/share"
      },
      "like": {
        "objectType": "EmbedActionHandler",
        "mediaType": "text/plain",
        "content": "Hello World"
      }
    }
  }
      

The "note" object has two declared actions, "share" and "like". Each of those is associated with one action handler object. The "share" action has a action handler of type "HttpActionHandler", while the "like" action has an "EmbedActionHandler".

As illustrated in the example, action handlers are represented as Activity Streams 2.0 objects. All such objects share a common set of base member properties as defined in the following table:

This specification defines three specific base types of action handler:

• The HTTP Action Handler [httpactionhandler],
• The Embed Action Handler [embedactionhandler], and
• The Intent Action Handler [intentactionhandler].

Implementations are free to use Activity Stream objects of any objectType as an action handler. Consuming applications MAY ignore any object it encounters that use objectTypes that are not recognized or supported as action handlers. Alternatively, the consuming application MAY treat such objects as implied Intent Action Handlers [intentactionhandler].

Multiple independent action handlers can be associated with any single verb using a JSON Array. The ordering of objects within such an array is not considered to be significant.

For example, in the following, the "share" action has two associated action handlers:

  {
    "objectType": "event",
    "displayName": "Party!",
    "content": "We're going to party like it's 1999!",
    "id": "urn:example:events:123",
    "actions": {
      "share": [
        {
          "objectType": "HttpActionHandler",
          "method": "GET",
          "url": "http://example.org/share-action",
          "target": "DIALOG",
          "returns": {
            "objectType": "TypedPayload",
            "mediaType": "text/html"
          }
        },
        {
          "objectType": "EmbedActionHandler",
          "mediaType": "text/html",
          "content": "<div>...</div>"
        }
      ]  
    }
  }
      

3. HTTP Action Handler

An HTTP Action Handler describes an HTTP request/response flow that is used to carry out an action. It is identified using an objectType value of "HttpActionHandler".

For example:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "view": {
        "objectType": "HttpActionHandler",
        "url": "http://example.org/foo",
        "method": "GET"
      }
    }
  }
      

As a shortcut, HttpActionHandlers that use the "GET" method and a "DEFAULT" target can be specified using a JSON string containing the absolute URL. For instance:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "view": "http://example.org/foo"
    }
  }
      

In the Activity Streams 2.0 format, the "url" property is defined as a "Link Value", this means that it is possible for the value of the "url" property to be an Activity Stream object that a consuming application can use to resolve the actual target URL. This specification defines a new UrlTemplate [urltemplate] objectType specifically intended for such use.

The UrlTemplate object can be used within an HTTP Action Handler, for instance, whenever carrying out the HTTP request requires the construction of a new URL that includes variable parameters:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "review": {
        "objectType": "HttpActionHandler",
        "url": {
          "objectType": "UrlTemplate",
          "method": "POST",
          "template": "http://example.org/note/123{?rating}",
          "parameters": {
            "rating": {
              "id": "http://schema.org/ratingValue",
              "displayName": "Rating",
              "maximum": 5,
              "minimum": 1,
              "format": "uint32"
            }
          }
        },
        "method": "GET",
        "target": "NEW"
      }
    }
  }
      

If the intended HTTP request uses the GET method and DEFAULT target, the UrlTemplate object itself can be used directly as the action handler.

"GET" HttpActionHandler shortcut using a URL Template:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "review": {
        "objectType": "UrlTemplate",
        "template": "http://example.org/note/123{?rating}",
        "parameters": {
          "rating": {
            "id": "http://schema.org/ratingValue",
            "displayName": "Rating",
            "maximum": 5,
            "minimum": 1,
            "format": "uint32"
          }
        }
      }
    }
  }
      

If the HTTP request requires an input payload, the HttpActionHandler object can contain an "expects" property. The value of "expects" is an Activity Streams 2.0 "Link Value" represented either as a simple JSON string containing a fully qualified IRI, an Activity Stream object, or an array of IRI's or Objects. This specification defines a new HtmlForm [htmlform] objectType to be used whenever the input of the HTTP request is an HTML Form POST. A new TypedPayload [typedPayload] objectType is defined for use whenever the input is an arbitrary MIME media type.

For example, the following describes an HTML Form post with a single "foo" parameter submitted using the "application/x-www-form-urlencoded" format:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "share": {
        "objectType": "HttpActionHandler",
        "method": "POST",
        "url": "http://example.org/foo",
        "expects": {
          "objectType": "HtmlForm",
          "mediaType": "application/x-www-form-urlencoded",
          "parameters": {
            "foo": {
              "id": "http://example.org/foo",
              "type": "http://www.w3.org/2001/XMLSchema#string",
              "displayName": "Foo Property"
            }
          }
        }
      }
    }
  }
      

4. Embed Action Handler

An Embed Action Handler defines static or dynamic content to be visually rendered to carry out an action. Examples of embeds can include static HTML, images, videos, gadgets and applications. It is identified using an objectType value of "EmbedActionHandler".

In the following example, the "view" action is associated with an "EmbedActionHandler" containing a static fragment of HTML markup:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "view": {
        "objectType": "EmbedActionHandler",
        "content": "<div>This is some bit of embedded HTML</div>",
        "mediaType": "text/html",
        "style": {
          "height": "100px",
          "width": "100px",
          "box-shadow": "10px 10px 5px #888888"
        },
        "displayName": "Some embedded content",
        "preview": "http://example.org/preview/123.jpg"
      }
    }
  }
      

Alternatively, the embedded content can be referenced by URL:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "view": {
        "objectType": "EmbedActionHandler",
        "url": "http://example.org/foo",
        "mediaType": "text/html"
      }
    }
  }
      

The mediaType parameter specifies the type of content to be embedded. Consuming applications MAY ignore Embed Action Handlers that specify unrecognized or unsupported mediaTypes.

Example:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "view": {
        "objectType": "EmbedActionHandler",
        "url": "http://example.org/foo.mpg",
        "mediaType": "video/mpeg"
      }
    }
  }
      

5. Intent Action Handler

An Intent Action Handler provides a generic way for the publisher of an Activity object to tell the consuming application to figure out how to handle the action on it's own. The consumer can, for instance, pass the object off to some other native platform application. It is identified using an objectType value of "IntentActionHandler".

For example:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "share": {
        "objectType": "IntentActionHandler",
        "displayName": "Share This",
        "context": {
          "foo": "ABC",
          "bar": 123
        }
      }
    }
  }
      

6. Using "service" and "application" objects as action handlers

The "service" and "application" object are existing objectTypes defined by the Activity Streams 1.0 core schema. While these objects were not originally designed to be used as action handlers, they can be. Specifically, the "service" objectType can be used when the action is to be carried out using some specific third party service interface; the "application" objectType can be used when the action is to be carried out by deferring some some specific native platform application.

For example:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "share": {
        "objectType": "service",
        "displayName": "My Sharing Service",
        "url": "http://share.example.org/api"
      },
      "save": {
        "objectType": "application",
        "displayName": "Read this later!",
        "platform": "android",
        "id": "123",
        "url": "http://play.google.com/..."
      }
    }
  }
      

7. HTML Form Objects

For example:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "review": {
        "objectType": "HttpActionHandler",
        "method": "POST",
        "url": "http://example.org/foo",
        "expects": {
          "objectType": "HtmlForm",
          "mediaType": "application/x-www-form-urlencoded",
          "parameters": {
            "foo": {
              "displayName": "Foo",
              "id": "http://example.org/FooProperty",
              "type": "http://www.w3.org/2001/XMLSchema#string",
              "required": True
            },
            "bar": {
              "displayName": "Bar",
              "id": "http://example.org/BarProperty",
              "type": "http://www.w3.org/2001/XMLSchema#",
              "required": True,
              "value": "Provided Value"
            }
          }
        }
      }
    }
  }
      

8. Typed Payload Objects

For example:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "review": {
        "objectType": "HttpActionHandler",
        "method": "POST",
        "url": "http://example.org/foo",
        "expects": {
          "objectType": "TypedPayload",
          "mediaType": "text/json",
        }
      }
    }
  }
      

9. URL Template Objects

Objects with the "UrlTemplate" object type represent [RFC6570] URL Templates.

{
  "objectType": "note",
  "displayName": "A simple note object",
  "content": "This is a simple note.",
  "actions": {
    "review": {
      "objectType": "UrlTemplate",
      "template": "http://example.org/foo/123{?rating}",
      "parameters": {
        "rating": {
          "displayName": "Rating",
          "id": "http://example.org/RatingProperty",
          "required": True
        }
      }
    }
  }
}
      

10. Parameters Object

A Parameters Object is used to provide descriptions of the variable inputs of objects such as HTML Forms [htmlform] and URL Templates [urltemplate]. The object is expressed as a JSON dictionary mapping parameter names to Type Values [I-D.snell-activitystreams] describing the parameters.

By default, all parameters defined within the object are assumed to be required. When a parameter is described using an Object, the object MAY contained a boolean "required" member. If "required" is false, use of the parameter is assumed to be optional.

Using the Parameters Object in UrlTemplate objects:

  {
    "objectType": "UrlTemplate",
    "template": "http://example.org{/foo,bar}"
    "parameters": {
      "foo": "http://example.org/FooProperty",
      "bar": {
        "id": "http://example.org/BarProperty",
        "displayName": "Bar",
        "required": False
      }
    }
  }
      

Using the Parameters Object in HtmlForm objects:

  {
    "objectType": "HtmlForm",
    "mediaType": "application/x-www-form-urlencoded",
    "parameters": {
      "foo": "http://example.org/FooProperty",
      "bar": {
        "objectType": "parameter",
        "id": "http://example.org/BarProperty",
        "displayName": "Bar",
        "required": False
      }
    }
  }
      

This specification defines a Parameter objectType [ParameterObject] specifically designed to describe parameters.

Implementations that encounter unrecognized or unexpected objectTypes used to describe parameters will likely be unable to successfully comprehend the parameter and may, therefore, be unable to carry out an Action.

10.1. Parameter Object

The "parameter" objectType is specifically intended for use with the Parameters [parameters] object. Each Parameter object provides a rich description of a single parameter.

The following common format strings MAY be used as values for the "format" property:

• "boolean" - Specifies that the value is either a string specifying "true" or "false", a boolean True or False, or an integer where 0 represents False and any other value represents True.
• "byte" - Specifies that the value is a "URL and filename safe" Base64-encoded string of bytes as defined by [RFC4648].
• "hex" - Specifies that the value is a String containing a sequence of Hex-encoded bytes.
• "date" - Specifies that the value is a "full-date" (YYYY-MM-DD) as defined by [RFC3339].
• "date-time" - Specifies that the value is a "date-time" as defined by [RFC3339].
• "double" - Specifies that the value is a double-precision 64-bit IEEE 754 floating point value.
• "duration" - Specifies that the value is a "duration" as defined by [RFC3339].
• "float" - Specifies that the value is a single-precision 32-bit IEEE 754 floating point.
• "int32" - Specifies that the value is a signed, 32-bit integer in the range -2,147,483,648 to 2,147,483,647 (inclusive).
• "int64" - Specifies that the value is a signed, 64-bit integer in the range -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 (inclusive).
• "uint32" - Specifies that the value is an unsigned 32-bit integer in the range 0 to 4,294,967,295 (inclusive).
• "uint64" - Specifies that the value is an unsigned 64-bit integer in the range 0 to (2^64)-1 (inclusive)
• "lang" - Specifies that the value is a Language Tag as defined by [RFC5646].
• "uri" - Specifies that the value is a string containing a URI as defined by [RFC3986].
• "iri" - Specifies that the value is a string containing an IRI as defined by [RFC3987].

Using the Parameter Object in HtmlForm objects:

  {
    "objectType": "HtmlForm",
    "mediaType": "application/x-www-form-urlencoded",
    "parameters": {
      "foo": "http://example.org/FooProperty",
      "bar": {
        "objectType": "parameter",
        "id": "http://example.org/BarProperty",
        "displayName": "Bar",
        "required": False,
        "repeated": False,
        "format": "uint32",
        "defaultValue": 3,
        "minimum": 1,
        "maximum": 5
      }
    }
  }
      

10.2. Using UrlTemplate and TypedPayload objects as parameter descriptions

In certain cases, when the value of a parameter is expected to be either a URI or IRI, the UrlTemplate objectType [urltemplate] MAY be used as the parameter description. In such cases, the "required", "repeated", "default" and "placeholder" properties from the Parameter objectType [ParameterObject] can be used as additional properties within the UrlTemplate object.

For example:

          {
            "objectType": "HtmlForm",
            "mediaType": "application/x-www-form-urlencoded",
            "parameters": {
              "foo": "http://example.org/FooProperty",
              "bar": {
                "objectType": "UrlTemplate",
                "id": "http://example.org/BarProperty",
                "template": "http://example.org{/foo}",
                "parameters": {
                  "foo": "http://example.org/#string"
                },
                "displayName": "Bar",
                "required": False,
                "repeated": False
              }
            }
          }
        

Likewise, when the value of a parameter is expected to be an instance of a specific MIME media type, the TypedPayload objectType [typedPayload] can be used.

          {
            "objectType": "HtmlForm",
            "mediaType": "multipart/form-data",
            "parameters": {
              "file": {
                "objectType": "TypedPayload",
                "mediaType": "image/*",
                "required": True,
                "repeated": True
              }
            }
          }
        

11. Authentication Object

An Authentication Object is used by Action Handlers that require specific authentication options to be supported in order to carry out the Action. The object is expresed as a JSON dictionary mapping authentication schema labels to JSON dictionaries that provide a specific description of properties and requirements specific to the scheme.

Example Authentication details:

  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note",
    "actions": {
      "view": {
        "objectType": "HttpActionHandler",
        "method": "GET",
        "url": "http://example.org/notes/1",
        "auth": {
          "basic": {
            "realm": "http://example.org"
          },
          "oauth": {
            "scopes": [
              "some.oauth.scope",
              "another.oauth.scope"
            ]
          }
        }
      }
    }
  }
      

This specification does not define the authentication schemes or their associated properties. Unrecognized authentication schemes MAY be ignored. However, if an implementation fails to recognize any of the authentication schemes specified by an Action Handler, it might not be possible to successfully carry out the Action.

12. Styles Object

A Styles Object is used by EmbedActionHandlers to provide CSS style hints for the container within which embedded content is to be displayed. The object is expressed as either a single JSON dictionary object mapping CSS property names to appropriate CSS values, or an array of JSON dictionary objects. An optional "media" member can be included within the dictionary providing a CSS Media Query.

Example style hints:

{
  "objectType": "note",
  "displayName": "A simple note object",
  "content": "This is a simple note.",
  "actions": {
    "view": {
      "objectType": "EmbedActionHandler",
      "content": "Some plain text content",
      "mediaType": "text/plain",
      "style": {
        "height": "100px",
        "width": "100px",
        "box-shadow": "10px 10px 5px #888888"
      }
    }
  }
}
      

Multiple style hints for specific media query targets:

{
  "objectType": "note",
  "displayName": "A simple note object",
  "content": "This is a simple note.",
  "actions": {
    "view": {
      "objectType": "EmbedActionHandler",
      "content": "Some plain text content",
      "mediaType": "text/plain",
      "style": [
        {
          "media": "print",
          "height": "100px",
          "width": "100px",
          "box-shadow": "10px 10px 5px #888888"
        },
        {
          "media": "screen and (orientation: landscape)",
          "height": "100px",
          "width": "100px",
          "box-shadow": "10px 10px 5px #888888"
        }
      ]
    }
  }
}
      

13. Security Considerations

TBD

14. IANA Considerations

TBD

15. Normative References

Appendix A. Using Action Handlers From Other Vocabularies

The Activity Streams 2.0 Actions mechanism is specifically designed to allow Action Handlers from multiple vocabularies.

A.1. Schema.org Actions Proposal

Based on http://www.w3.org/wiki/images/b/b9/Actionsinschema.org.pdf:

  {
    "objectType": "video",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "watch": [
        {
          "objectType": "http://schema.org/WebPageHandler",
          "url": "http://movies.example.com/player?id=123" 
        },
        {
          "objectType": "http://schema.org/AndroidHandler",
          "url": "http://movies.example.com/player?id=123",
          "package": "com.movies"
        }
      ]
    }
  }
        

A.2. Google's "Actions in the Inbox"

Based on https://developers.google.com/gmail/actions/reference/review-action:

      
  {
    "objectType": "note",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "review": {
        "objectType": "http://schema.org/ReviewAction",
        "review": {
          "objectType": "http://schema.org/Review",
          "itemReviewed": {
            "objectType": "http://schema.org/FoodEstablishment",
            "name": "Joe's Diner"
          },
          "reviewRating": {
            "objectType": "http://schema.org/Rating",
            "bestRating": "5",
            "worstRating": "1"
          }
        },
        "handler": {
          "objectType": "http://schema.org/HttpActionHandler",
          "url": "http://reviews.com/review?id=123",
          "requiredProperty": {
            "objectType": "http://schema.org/Property",
            "name": "review.reviewRating.ratingValue"
          },
          "method": "http://schema.org/HttpRequestMethod/POST"
        }
      }
    }
  }
        

A.3. Mixing Vocabularies

  {
    "objectType": "video",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "watch": [
        {
          "objectType": "HttpActionHandler",
          "url": "http://movies.example.com/player?id=123", 
          "target": "NEW"
        },
        {
          "objectType": "http://schema.org/AndroidHandler",
          "url": "http://movies.example.com/player?id=123",
          "package": "com.movies"
        }
      ]
    }
  }
        

A.4. Example Drawing From Multiple Vocabularies

  {
    "objectType": "video",
    "displayName": "A Movie!",
    "displayName": "A simple note object",
    "content": "This is a simple note.",
    "actions": {
      "watch": [
        {
          "objectType": "EmbedActionHandler",
          "displayName": "HD",
          "mediaType": "video/mpeg",
          "url": "http://cdn.example.org?id=123amp;fmt=HD",
        },
        {
          "objectType": "EmbedActionHandler",
          "displayName": "SD",
          "mediaType": "video/mpeg",
          "url": "http://cdn.example.org?id=123&fmt=SD",
        },
        {
          "objectType": "application",
          "displayName": "Watch on Netflix",
          "url": "http://netflix.com..."
        }
      ],
      "like": {
        "objectType": "EmbedActionHandler",
        "mediaType": "text/html",
        "url": "http://www.facebook.com/plugins/like.php...",
        "style": {
          "width": "150px",
          "height": "50px"
        }
      },
      "share": [
        {
          "objectType": "HttpActionHandler",
          "displayName": "Twitter",
          "url": "https://twitter.com/share?url=...",
          "target": "DIALOG"
        },
        {
          "objectType": "HttpActionHandler",
          "displayName": "Facebook",
          "url": "https://www.facebook.com/sharer/sharer.php?u=...",
          "target": "DIALOG"
        }
      ],
      "save": [
        {
          "objectType": "service",
          "id": "http://getpocket.com",
          "displayName": "Pocket",
          "context": {
            "url": "http://example.org/movie?id=123",
            "title": "A Movie!",
            "tags": "foo, bar, baz"
          }
        },
        {
          "objectType": "service",
          "id": "http://instapaper.com",
          "displayName": "Instapaper",
          "context": {
            "url": "http://example.org/movie?id=123",
            "title": "A Movie!",
            "selection": "An action movie!"
          }
        }
      ],
      "review": {
        "objectType": "HttpActionHandler",
        "displayName": "Rate this movie!",
        "url": "http://review.example.org/movie?id=123",
        "method": "POST",
        "expects": {
          "objectType": "HtmlForm",
          "mediaType": "application/x-www-form-urlencoded",
          "parameters": {
            "rating": {
              "id": "http://schema.org/ratingValue",
              "maximum": 5,
              "minimum": 0,
              "format": "uint32",
              "displayName": "Rating",
              "required": true
            },
            "comments": {
              "id": "http://schema.org/commentText",
              "displayName": "Comments",
              "required": "false"
            }
          }
        }
      }
    }
  }
        

Authors' Addresses