Speed up -- use littler?

- http://dirk.eddelbuettel.com/blog/2014/09/02/#littler-faster-at-doing-nothing
- http://dirk.eddelbuettel.com/code/littler.html
- https://cran.r-project.org/web/packages/littler/vignettes/littler-examples.html
- https://cran.r-project.org/web/packages/littler/vignettes/littler-faq.html
