public Builder observer(final DoubleConsumer observer) {
			this.observer = requireNonNull(observer);
			return this;
		}
Set an additional value observer.

@param observer the value observer
@throws NullPointerException if the given {@code observer} is
        {@code null}.
@return {@code this} builder
jenetics.incubator/src/main/java/io/jenetics/incubator/stat/Histogram.java