@Contract("null, _ -> null")
  @Nullable
  public NullabilityProblem<T> problem(@Nullable T anchor, @Nullable PsiExpression expression) {
    return anchor == null || this == noProblem ? null : new NullabilityProblem<>(this, anchor, expression, false);
  }
@Contract("null, _ -> null")
  @Nullable
  public NullabilityProblem<T> problem(@Nullable T anchor, @Nullable PsiExpression expression)