def CustomMergingCollection
  include Enumerable

  def initialize(hash={})
    @hash = hash
  end

  def each(&block)
    @hash.values.each(&block)
  end

  def []=(other)
    idx = other.idx
    return @hash[idx] = other unless @hash[idx]
    @hash[idx] = combine(current, other)
  end

  private

  def combine(first, second)
    # ...
  end
end