anchor.diff:  a particular class for documentation purposes.
   Use `Workflow::create_workflow_diagram(class)` in your rake task like:
   
  -    namespace :doc do
  -      desc "Generate a workflow graph for a model passed e.g. as 'MODEL=Order'."
  -      task :workflow => :environment do
  -        require 'workflow/draw'
  -        Workflow::Draw::workflow_diagram(ENV['MODEL'].constantize)
  -      end
  -    end
  -
  +```rb
  +namespace :doc do
  +  desc "Generate a workflow graph for a model passed e.g. as 'MODEL=Order'."
  +  task :workflow => :environment do
  +    require 'workflow/draw'
  +    Workflow::Draw::workflow_diagram(ENV['MODEL'].constantize)
  +  end
  +end
  +```
   
   
   Changelog
  
positive.code: overrides the persistence methods `load_workflow_state` and
  `persist_workflow_state`. Example:
  
      module Workflow
        module SuperCoolDb
          module InstanceMethods
            def load_workflow_state
              # Load and return the workflow_state from some storage.
              # You can use self.class.workflow_column configuration.
            end
  
            def persist_workflow_state(new_value)
              # save the new_value workflow state
            end
          end
  
          module ClassMethods
            # class methods of your adapter go here
          end
  
          def self.included(klass)
            klass.send :include, InstanceMethods
            klass.extend ClassMethods
          end
        end
      end
  
  The user of the adapter can use it then as:
  
      class Article
        include Workflow
        include Workflow:SuperCoolDb
        workflow do
          state :submitted
          # ...
        end
      end
  
  I can then link to your implementation from this README. Please let me
  also know, if you need any interface beyond `load_workflow_state` and
  
negative.code: Workflow::TransitionHalted exception. You can provide an additional
  `halted_because` parameter.
  
  ```rb
  def reject(reason)
    halt! 'We do not reject articles unless the reason is important' \
      unless reason =~ /important/i
  end
  ```
  
  The traditional `halt` (without the exclamation mark) is still supported
  too. This just prevents the state change without raising an
  