Mountpoints collection objects where the content of the Mountpoints are located
in another collection, in most cases in another mailbox.  Mountpoints are
identified with <Y:mountpoint/> in resourcetype property.

Also the following WebDAV attributes are set on the Mountpoint objects.

mountpoint-target-privilege-set
  - stores the ACL for the current user
  
mountpoint-target-URL
  - contains the URL to the target collection that owns the content of the 
    mountpoint

The new attributes mountpoint, mountpoint-target-privilege-set and 
mountpoint-target-URL are declared in Yahoo namespace "http://yahoo.com/ns/".

Example WebDAV requests and responses:

PROPFIND /dav/Jong/ HTTP/1.1
Depth: 1
Content-Type: application/xml; charset="utf-8"
Content-Length: nnn

<?xml version="1.0" encoding="utf-8"?>
<D:propfind xmlns:D="DAV:" xmlns:Y="http://yahoo.com/ns/">
  <D:prop>
    <D:resourcetype/>
    <D:current-user-privilege-set/>
    <Y:mountpoint-target-privilege-set/>
    <Y:mountpoint-target-URL/>
  </D:prop>
</D:propfind>


HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx

<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav" 
               xmlns:Y="http://yahoo.com/ns/">
  <D:response>
    <D:href>/dav/Jong/Holidays/</D:href>
    <D:propstat>
      <D:prop>
        <D:resourcetype>
          <D:collection/>
          <C:calendar/>
          <Y:mountpoint/>
        </D:resourcetype>
        <D:current-user-privilege-set>
          <D:privilege><D:read/></D:privilege>
          <D:privilege><D:write-properties/></D:privilege>
        </D:current-user-privilege-set>
        <Y:mountpoint-target-privilege-set>
          <D:privilege><D:read/></D:privilege>
        </Y:mountpoint-target-privilege-set>
        <Y:mountpoint-target-URL>
          <D:href>/dav/HolidayCalendars/US/</D:href>
        </Y:mountpoint-target-URL>
      </D:prop>
      <D:status>HTTP/1.1 200 OK</D:status>
    </D:propstat>
  </D:response>
  <D:response>
    <D:href>/dav/Jong/FamilyCalendar/</D:href>
    <D:propstat>
      <D:prop>
        <D:resourcetype>
          <D:collection/>
          <C:calendar/>
          <Y:mountpoint/>
        </D:resourcetype>
        <D:current-user-privilege-set>
          <D:privilege><D:read/></D:privilege>
          <D:privilege><D:write-properties/></D:privilege>
        </D:current-user-privilege-set>
        <Y:mountpoint-target-privilege-set>
          <D:privilege><D:read/></D:privilege>
          <D:privilege><D:write-content/></D:privilege>
        </Y:mountpoint-target-privilege-set>
        <Y:mountpoint-target-URL>
          <D:href>/dav/GroupCalendars/JongLee/</D:href>
        </Y:mountpoint-target-URL>
      </D:prop>
      <D:status>HTTP/1.1 200 OK</D:status>
    </D:propstat>
  </D:response>
</D:multistatus>


Alternatively the client can request the ACL for the current principal directly
from the shared calendar.

PROPFIND /dav/HolidayCalendars/US/ HTTP/1.1
Depth: 0
Content-Type: application/xml; charset="utf-8"
Content-Length: nnn

<?xml version="1.0" encoding="utf-8"?>
<D:propfind xmlns:D="DAV:">
  <D:prop>
    <D:resourcetype/>
    <D:current-user-privilege-set/>
  </D:prop>
</D:propfind>

HTTP/1.1 207 Multi-Status
Content-Type: text/xml; charset="utf-8"
Content-Length: xxx

<?xml version="1.0" encoding="utf-8" ?>
<D:multistatus xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
  <D:response>
    <D:href>/dav/HolidaysCalendars/US/</D:href>
    <D:propstat>
      <D:prop>
        <D:resourcetype>
          <D:collection/>
          <C:calendar/>
        </D:resourcetype>
        <D:current-user-privilege-set>
          <D:privilege><D:read/></D:privilege>
        </D:current-user-privilege-set>
      </D:prop>
      <D:status>HTTP/1.1 200 OK</D:status>
    </D:propstat>
  </D:response>
</D:multistatus>