module Enumerable

def slice_after(pattern = Backports::Undefined, &block)
  raise ArgumentError, 'both pattern and block are given' if pattern != Backports::Undefined && block
  raise ArgumentError, 'wrong number of arguments (given 0, expected 1)' if pattern == Backports::Undefined && !block
  enum = self
  block ||= proc {|elem| pattern === elem}
  Enumerator.new do |y|
    acc = []
    enum.each do |*elem|
      elem = elem.first if elem.length == 1
      acc << elem
      if block.call(elem)
        y.yield acc
        acc = []
      end
    end
    y.yield acc unless acc.empty?
  end
end