class Symbol: A +Symbol+ object represents a named identifier inside the Ruby interpreter. literals.| docs.ruby-lang.org
class Proc: A +Proc+ object is an encapsulation of a block of code, which can be stored in a local variable, passed to a method or another Proc, and can be called| docs.ruby-lang.org
class Hash: A +Hash+ maps each of its unique keys to a specific value. An Array index is always an Integer.| docs.ruby-lang.org
module Kernel: The Kernel module is included by class Object, so its methods are available in every Ruby object. The Kernel instance methods are documented in class| docs.ruby-lang.org