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### Helper code

class Context

  attr_reader :content

  def initialize(content)
    @content = content
  end

  def replace(old_string, new_string)
    @content.gsub!(old_string, new_string)
  end

end

def magic(content, &block)
  Context.new(content).instance_eval(&block)
end

### Actual code

stuff = 'blah blaaah blah'
magic(stuff) do
  replace 'blaaah', 'moo'
  content.upcase!
end

puts stuff.inspect