default PARENT getParentOrThrow() {
        return getParent().orElseThrow(
            () -> new IllegalStateException(
                "Unable to get parent for " + toString()
            )
        );
    }
Returns the parent of this document or throws an exception if this was
the root. This method can be used if you know beforehand that this
document is never the root.

@return                        the parent document
@throws IllegalStateException  if this was the root
@see #getParent()
runtime-parent/runtime-config/src/main/java/com/speedment/runtime/config/trait/HasParent.java