@Nullable
  public Mutability meet(@NotNull Mutability other) {
    if (this == other) return this;
    if (this == UNKNOWN) return other;
    if (other == UNKNOWN) return this;
    if (this == MUTABLE || other == MUTABLE) return null;
    if (this == UNMODIFIABLE || other == UNMODIFIABLE) return UNMODIFIABLE;
    if (this == UNMODIFIABLE_VIEW || other == UNMODIFIABLE_VIEW) return UNMODIFIABLE_VIEW;
    return MUST_NOT_MODIFY;
  }
@Nullable
  public Mutability meet(@NotNull Mutability other)