hash_purge_if.rb

require 'active_support/concern'

module Hash::PurgeIf
  include ActiveSupport::Concern
  
  included do |base|
    raise 'Hash#purge_if already defined' if
      base.instance_methods.include?(:purge_if)
  end
  
  def purge_if(...)
    ...
  end
end

class Hash
  include Hash::PurgeIf
end