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What is Ruby recursion and how does it work?
>> def countdown(n)
..   return if n.zero? # base case
..   puts n
..   countdown(n-1)    # getting closer to base case
..   end #=> nil
>> countdown(5) #=> nil
5
4
3
2
1

def fib(n)
  return n if (0..1).include? n
  fib(n-1) + fib(n-2) if n > 1
end

h = {foo: 42, bar: 666}
parent = {child: {foo: 42, bar: 666}}
h[:parent] = parent
h.inspect # => {:foo=>42, :bar=>666, :parent=>{:child=>{...}}}

x = []
y = [x]
x << y
x.inspect # => [[[...]]]
x == [x]  # => true