import java.util.*;
import java.util.zip.*;
import java.util.List;
import java.util.regex.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.concurrent.locks.*;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.text.*;
import javax.swing.table.*;
import java.io.*;
import java.net.*;
import java.lang.reflect.*;
import java.lang.ref.*;
import java.lang.management.*;
import java.security.*;
import java.security.spec.*;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import javax.imageio.*;
import java.math.*;
// could almost add IVar<A> here - but void set() and bool set() are incompatible in some places
class main {

static interface IRef<A> extends IF0<A> {
  // called by the referencee to atomically replace itself
  // Passing oldValue to avoid race conditions
  public default void replaceValue(A oldValue, A newValue) {}
}
static interface IF0<A> {
  A get();
}

}