The Packages section in Chapter 1, Installing the Julia Platform, introduced us to Julia's package system (some 1,906 packages in September 2018 and counting) and its manager program Pkg. Most Julia libraries are written exclusively in Julia; this makes them not only more portable, but also an excellent source for learning and experimenting with Julia in your own modified versions. The packages that are useful for data scientists are Stats, Distributions, GLM, and Optim. You can search for applicable packages in the https://pkg.julialang.org/ repository.