@Override
	public String toString() {
		return format("%s -> %s", _left, _right);
	}
Maps {@code this} rewrite rule from type {@code V} to type {@code B}.

@param mapper the type mapper
@param <B> the target type
@return a new rewrite rule for the mapped type
@throws NullPointerException if the {@code mapper} is {@code null}
jenetics.ext/src/main/java/io/jenetics/ext/rewriting/TreeRewriteRule.java