@Override
    public MapStream<K, V> sequential() {
        inner = inner.sequential();
        return this;
    }
Returns an equivalent stream that is sequential.  May return
itself, either because the stream was already sequential, or because
the underlying stream state was modified to be sequential.
<p>
This is an intermediate operation.

@return  a sequential stream
common-parent/mapstream/src/main/java/com/speedment/common/mapstream/MapStream.java