code: @SuppressWarnings("unchecked")
  	public Mapper<K, V> keyAndValueNotNull() {
  		return addConstraint((MapConstraint<K, V>) MapConstraints.notNull());
  	}
docs: Add a constraint that verifies that neither the key nor the value is
  null. If either is null, a {@link NullPointerException} is thrown.
  
  @return