@NotNull
  public DfaNullability unite(@NotNull DfaNullability other) {
    if (this == other) {
      return this;
    }
    if (this == NULL || other == NULL ||
        this == NULLABLE || other == NULLABLE) {
      return NULLABLE;
    }
    if (this == FLUSHED || other == FLUSHED) {
      return FLUSHED;
    }
    return UNKNOWN;
  }
@NotNull
  public DfaNullability unite(@NotNull DfaNullability other)