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

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