class A
  def initialize
    @f = 5
  end
  def f
    @f
  end
  def f= x
    @f = x
  end
end

a = A.new
b = a
c = A.new

a.f = 7
x = a.f
a.f = c
x = a.f
x.f = c


class Frac

public # implicit, anyone can call

  def initialize(num, den=1)
    if den == 0
      raise "bogus"
    elsif den < 0
      @num = - num
      @den = - den
    else
      @num = num
      @den = den
    end
  end

  def to_s
    if @den == 1
      @num.to_s
    else
      @num.to_s + "/" + @den.to_s
    end
  end

  def add! r
    @num = (@num * r.den) + (r.num * @den)
    @den = @den * r.den
    self
  end

  def + r
    Frac.new(@num, @den).add! r
  end

protected # any method in the same class can call

  def den
    @den
  end

  def num
    @num
  end

  def foo
    reduce
    # self.reduce # won't work
  end

private # method can only call on itself

  def reduce
    self
  end
end