private T advanceThis() {
            return buffer.get(nextIndex++);
        }
Buffers (i.e. advances the <code>source</code>) until an item at <code>nextIndex</code>
is added to the <code>buffer</code>, or until the <code>source</code> is exhausted.

Guarded by: <code>buffer</code>
jOOL-java-8/src/main/java/org/jooq/lambda/SeqBuffer.java