/**
* Takes an <code>Either</code> to its contained value within left or right.
*
* @param e The either to reduce.
* @return An <code>Either</code> to its contained value within left or right.
*/
public static <A> A reduce(final Either<A, A> e) {
return e.isLeft() ? e.left().value() : e.right().value();
}
/**
* Returns the value of this projection or fails with the given error message.
*
* @param err The error message to fail with.
* @return The value of this projection
*/
public A valueE(final P1<String> err) {
if (e.isLeft())
//noinspection CastToConcreteClass
return ((Left<A, B>) e).a;
else throw error(err._1());
}