- A[A == 1] should work on a 1D DelayedArray object (and return a 1D
  DelayedArray object).

- Add unit tests for all the DelayedOp types.

- Should backend classes (e.g. HDF5Array) be defined as:
    setClass("HDF5Array",
        contains="DelayedArray",
        representation(seed="HDF5ArraySeed")
    )
  Then validity method is not needed anymore.

- Document global option DelayedArray.block.size

- Add man page and unit tests for statistical methods defined in
  DelayedArray-stats.R

- Support delayed array replication.
  See https://github.com/Bioconductor/DelayedArray/issues/8

- Make DelayedArray contain Annotated from S4Vectors?

- Add more examples to the man pages (using the toy dataset).

- Add unit tests for round() and signif() (Math2 group).

- Explore DelayedAtomicVector. Could be used in situations where
  1D DelayedArray objects are currently used (e.g. VariantExperiment
  package). Then reconsider what subsetting does when 'drop' is TRUE.
  Should it return the result as an ordinary vector instead of a 1D
  DelayedArray object? We're not doing this right now because it would
  be inconvenient for people using 1D DelayedArray objects (it triggers
  realization when the user probably doesn't want it). But if people
  switch to DelayedAtomicVector objects then maybe it's ok to make this
  change.

- Support subsetting an arbitrary object by a DelayedArray or
  DelayedMatrix of type logical.

- Support more matrix- and array-like operations.

- How well supported are DelayedArray of type "character"?

- Add more unit tests.

