extra stage https://gist.github.com/mame/357bab3d5dfb8e830c8656f07d28a468

extra-1.rb

Hello = "Hello"

# Hint: Stop the recursion.
def Hello
  Hello%() +
    " world"
end

puts Hello()

extra-2.rb

s = "Dz"
# Hint: https://techlife.cookpad.com/entry/2018/12/25/110240
s == s.upcase or
s == s.downcase or puts "Hello world"

extra-3.rb

def say
  s %= 'Small'
  t = 'world'
  puts "#{s} #{t}"
end

TracePoint.new(:line){|tp|
  tp.binding.local_variable_set(:s, 'Hello')
  tp.binding.local_variable_set(:t, 'Ruby')
  tp.disable
}.enable(target: method(:say))

say

extra-3_b.rb

def say\
  s = 'Small'
  t = 'world'
  puts "#{s} #{t}"
end

TracePoint.new(:line){|tp|
  tp.binding.local_variable_set(:s, 'Hello')
  tp.binding.local_variable_set(:t, 'Ruby')
  tp.disable
}.enable(target: method(:say))

say