code: function( elems, callback, arg ) {
  		var value,
  			i = 0,
  			length = elems.length,
  			isArray = isArraylike( elems ),
  			ret = [];
  
  		// Go through the array, translating each of the items to their
  		if ( isArray ) {
  			for ( ; i < length; i++ ) {
  				value = callback( elems[ i ], i, arg );
  
  				if ( value != null ) {
  					ret[ ret.length ] = value;
  				}
  			}
  
  		// Go through every key on the object,
  		} else {
  			for ( i in elems ) {
  				value = callback( elems[ i ], i, arg );
  
  				if ( value != null ) {
  					ret[ ret.length ] = value;
  				}
  			}
  		}
  
  		// Flatten any nested arrays
  		return core_concat.apply( [], ret );
  	}
docs: arg is for internal usage only