# We can add methods to any class at any point. 
# This technique is called "mix-ins".
#  
#  (We can even add methods to individual objects; see 
#   https://blog.jcoglan.com/2013/05/08/how-ruby-method-dispatch-works/
#   about "singleton classes").

class Array; 
  def cdr 
    self[1..-1]
  end

  def null?
    self.size == 0
  end 
end


# Examine  Array.instance_methods  before and after running this code.
#   --You should see :cdr and :null? added to the method list.  

#---cut here initially when testing this file---

# Testing in IRB:  
# load "add-cdr-to-array.rb"
# a = [1, 2, 3]
# a.cdr.cdr.cdr.cdr.null?

# The latter fails! The fourth cdr returns nil, which is of NilClass
#   (just see nil.class). This class has no null? method defined!
# Let's add it:

class NilClass;
  def null?
    true
  end
end

# Now a.cdr.cdr.cdr.cdr.null? succeeds with 'true'
# Check  nil.class.instance_methods  before and after this addition.