code: @SuppressWarnings("null")
    public static
    @NonNull
    <T> T notNull(
      final @Nullable T x,
      final @NonNull String message)
    {
      if (x == null) {
        final StringBuilder text = new StringBuilder(32 + message.length());
        text.append("Expression evaluated to null: ");
        text.append(message);
        throw new NullCheckException(text.toString());
      }
      return x;
    }
docs: Check that {@code x} is not null, raising
  {@link NullCheckException} iff {@code x == null}.
  
  @param <T>     The type of values
  @param x       An arbitrary value
  @param message A descriptive message describing the value
  
  @return {@code x}