For release 0.7:
  - implement further "lexical constants" as productivity measures, with improved documentation
  - also include measures that require token vector (such as MTLD)?
  - add code examples for lnre.bootstrap() (once implementation of confint estimators and productivity measures is complete)
  - add examples for productivity.measures()

Further tasks:  
  - revise all built-in data sets
    - update internal format (e.g. avoid factors, make N numeric rather than integer, etc) so they are identical after round-trip through disk file
    - convert all factors (for types) to character
    - verify that encodings for non-ASCII data are set appropriately
    - save with xz compression, especially for larger data sets
    - but make sure to use version=2 so the package is still compatible with R < 3.5.0
  - implement lnre.productivity.measures()
    - empirical bootstrap estimates should be fairly easy
    - approximate expectation and variance estimates where the mathematics can be worked out
    - don't forget to remove from .Rbuildignore then!
  - complete implementation of posterior type distributions for ZM and fZM models (after writing down the mathematical equations precisely)
  - custom code for posterior density, extended to m=0 for models with finite S
  - model-based Good-Turing estimates with function gtlnre()
  - improve performance of sample.tfl / zipfR.tflsplit with rmvhyper (from package extraDistr)?
    - adds dependency on a heavy Rcpp-based package only because of a rarely-used function
    - alternative: re-implement incremental splitting procedure with rhyper() in C
