Binary package “ri” in ubuntu xenial

Ruby Interactive reference

 ri is a command line tool that displays descriptions of built-in Ruby
 methods, classes, and modules. For methods, it shows you the calling
 sequence and a description. For classes and modules, it shows a
 synopsis along with a list of the methods the class or module
 This package provides ri command and descriptions about Ruby.
 This package provides the ri documentation for the Ruby standard library.
 The ri executable itself is in the ruby package.
 This package is a dependency package, which depends on Debian's default Ruby
 version (currently v2.3).