Ruby Interactive reference (for Ruby 1.9.1)

 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
 implements.
 .
 This package provides ri command and descriptions about Ruby 1.9.1.
 This package provides the ri documentation for the Ruby 1.9.1 standard library.
 The ri1.9.1 executable itself is in the ruby1.9.1 package.