private boolean canAdvanceThisWithBuffering() {
            return canAdvanceThisAtOnce() || tryAdvanceSource() && canAdvanceThisAtOnce();
        }
Tries to advance this <code>Spliterator</code> to element at <code>nextIndex</code>,
buffering <code>source</code> elements into <code>buffer</code> if necessary.

Synchronized on <code>buffer</code> in order to:
- obtain accurate <code>buffer.size()</code>
- safely copy from <code>source</code> to <code>buffer</code> (if needed)
- safely call <code>buffer.get()</code>
jOOL/src/main/java/org/jooq/lambda/SeqBuffer.java