Encapsulated Classes with Reference Semantics
person("Winston", "Chang", role = c("aut", "cre"), email = "winston@stdout.org")
Creates classes with reference semantics, similar to R's built-in
    reference classes. Compared to reference classes, R6 classes are simpler
    and lighter-weight, and they are not built on S4 classes so they do not
    require the methods package. These classes allow public and private
    members, and they support inheritance, even when the classes are defined in
    different packages.
knitr, microbenchmark, pryr, testthat, ggplot2, scales