gistfile1.rb

class A
  def foo
    puts 'original'
  end
  
  alias bar foo
  
  def foo
    puts 'modified'
  end
end

A.new.bar # => "original"

# -------------------------------------

class A
  def foo
    puts 'original'
  end
end

class B < A
  alias bar foo
end

class A
  def foo
    puts 'modified'
  end
end

B.new.bar # => "original"