Array#=~ メソッドはこういう挙動であるべき的な

findnum.rb

class Object

  def in? enum
    enum.include? self
  end

end

module Enumerable

  def =~ _
    if _.kind_of? self.class
      duplicate _
    else
      include? _
    end
  end

  def duplicate other
    i = 0
    each do |mine|
      if mine == other.first
        matched = true
        other.zip(last count - i) do |theirs, matchers|
          break matched = false unless theirs == matchers
        end
        return i if matched
      end
      i += 1
    end
    nil
  end

end

puts [1, 2, 3] =~ [1, 2] # => 0
puts [1, 2, 3] =~ [2, 3] # => 1
puts [1, 2, 3] =~ [1, 3] # => nil
puts [1, 2, 3] =~ 1 # => true # indexと同じのほうがいいかも
puts [1, 2, 3] =~ 0 # => false # nil のがいいかも