code: function hashFragmentFor (webId, resourceUrl,
    authType = acl.ACCESS_TO) {
    let hashKey = webId + '-' + resourceUrl + '-' + authType
    return hashKey
  }
docs: --- Standalone (non-instance) functions -- 
  Utility method that creates a hash fragment key for this permission.
  Used with graph serialization to RDF, and as a key to store permissions
  in a PermissionSet. Exported (mainly for use in PermissionSet).
  @method hashFragmentFor
  @param webId {String} Agent or group web id
  @param resourceUrl {String} Resource or container URL for this permission
  @param [authType='accessTo'] {String} Either 'accessTo' or 'default'
  @return {String}