exercise5_by_ray_attempt1.rb

# MaskedString = Class.new(String)
Object.const_set("MaskedString", Class.new(String))

MaskedString.instance_eval do
  define_method :tr_vowel do
    tr 'aeiou', '*'
  end
end

MaskedString.class_eval do
  define_singleton_method :tr_vowel do |str|
    str.tr 'aeiou', '*'
  end
end

puts MaskedString.new("Rainer").tr_vowel
puts MaskedString.tr_vowel("Rainer")