code: function _collectURLs ( token ) {
  
          var elem, isArrayElem;
  
          if ( Array.isArray( token ) ) {
  
              for ( var i = 0; i < token.length; i++ ) {
  
                  elem = token[ i ];
                  isArrayElem = Array.isArray( elem );
  
                  if ( isArrayElem && ( elem[ 0 ] === 'uri' ) ) {
  
                      urls.push( elem );
  
                  } else if ( isArrayElem ) {
  
                      _collectURLs( elem );
  
                  }
  
              }
  
          }
  
      }
docs: collection of urls to work with 
  Generates initial document tree.
  
  @private