code: @SuppressWarnings("unchecked")
  	public Mapper<K, V> keyNotNull() {
  		return addConstraint((MapConstraint<K, V>) KeyNotNullRestraint.INSTANCE);
  	}
docs: Add a constraint that verifies that the key is
  null. If is null, a {@link NullPointerException} is thrown.
  
  @return