module Enumerable

def slice_when(&block)
  raise ArgumentError, 'tried to create Proc object without a block' unless block
  enum = self
  Enumerator.new do |y|
    acc = []
    prev = Backports::Undefined
    enum.each do |*elem|
      elem = elem.first if elem.length == 1
      unless prev == Backports::Undefined
        if block.call(prev, elem)
          y.yield acc
          acc = []
        end
      end
      acc << elem
      prev = elem
    end
    y.yield acc unless acc.empty?
  end
end