public void back( int len ) {
        if ( pollIndex >= len )
            pollIndex -= len;
        else
            pollIndex = pollIndex + capacity() - len;

    }
public void back( int len ) {
        if ( pollIndex >= len )
            pollIndex -= len;
        else
            pollIndex = pollIndex + capacity() - len;

    }
public void back ( int len ) { if ( pollIndex >= len ) pollIndex -= len ; else pollIndex = pollIndex + capacity ( ) - len ; }
'unread' len bytes
@param len
unread len bytes