@NotNull
  public DfReferenceType asDfType() {
    return switch (this) {
      case NULL -> DfTypes.NULL;
      case NOT_NULL -> DfTypes.NOT_NULL_OBJECT;
      case UNKNOWN -> DfTypes.OBJECT_OR_NULL;
      default -> DfTypes.customObject(TypeConstraints.TOP, this, Mutability.UNKNOWN, null, DfType.BOTTOM);
    };
  }
@NotNull
  public DfReferenceType asDfType()