anchor.diff:  class MyHashToMessagePackMapper < RubyEventStore::Mappers::PipelineMapper
     def initialize
       super(RubyEventStore::Mappers::Pipeline.new(
  -      transformations: [
  -        MessagePackSerialization.new,
  -      ]
  +      MessagePackSerialization.new
       ))
     end
   end
  
positive.code: require 'msgpack'
  
  class MessagePackSerialization
    def dump(item)
      RubyEventStore::Mappers::Transformation::Item.new(
        event_id:   item.event_id,
        metadata:   item.metadata.to_msg_pack,
        data:       item.data.to_msg_pack,
        event_type: item.event_type
      )
    end
  
    def load(item)
      RubyEventStore::Mappers::Transformation::Item.new(
        event_id:   item.event_id,
        metadata:   MessagePack.unpack(item.metadata),
        data:       MessagePack.unpack(item.data),
        event_type: item.event_type
      )
    end
  end
  
negative.code: 
  ## Custom mapper
  
  Mapper is defined as a pipeline of transformations that transforms domain event object into record and back from record to domain event. I.e. `Default` is implemented as a set of 2 transformations. Transformation works on [Record](https://github.com/RailsEventStore/rails_event_store/blob/master/ruby_event_store/lib/ruby_event_store/record.rb) objects. Such `Record` is given to transformation methods (it is expected that `load` & `dump` methods are implemented) and it is result of any transformation performed. Except transformations, each `PipelineMapper` needs to be given 1 edge transformation:
  
  * to transform domain event to/from record (default [`RubyEventStore::Mappers::Transformation::DomainEvent`](https://github.com/RailsEventStore/rails_event_store/blob/master/ruby_event_store/lib/ruby_event_store/mappers/transformation/domain_event.rb))
  
  Extended implementation of [Default](https://github.com/RailsEventStore/rails_event_store/blob/master/ruby_event_store/lib/ruby_event_store/mappers/default.rb) with explicit default arguments:
  
  ```ruby
  module RubyEventStore
    module Mappers
      class Default < PipelineMapper
        def initialize(events_class_remapping: {})
          super(Pipeline.new(
            Transformation::EventClassRemapper.new(events_class_remapping),
            Transformation::SymbolizeMetadataKeys.new,
            to_domain_event: Transformation::DomainEvent.new
          ))
        end
      end
  