I have a simpler version:

```ruby
# frozen_string_literal: true

module Biz
  module Periods
    class Linear < SimpleDelegator

      def initialize(periods, shifts, selector)
        @periods   = periods.to_enum
        @shifts    = shifts.to_enum
        @selector  = selector
        @sequences = [@periods, @shifts]

        @inner_repr =
          Enumerator.new do |yielder|
            loop do
              periods.next and next if periods.peek.date == shifts.peek.date

              yielder << begin
                sequences
                  .public_send(selector) { |sequence| sequence.peek.date }
                  .next
              end
            end

            loop do yielder << periods.next end
          end

        super(@inner_repr)
      end

      private

      attr_reader :periods,
                  :shifts,
                  :selector,
                  :sequences

      def to_a
        @periods.to_a
        @shifts.to_a

        @inner_repr.to_a
      end

    end
  end
end
```