```ruby
require 'backports'
require 'adamantium'
require 'equalizer'
require 'abstract_type'
require 'concord'

# Namespace module
module ROM

  # Session namespace
  class Session

    # Error raised on illegal state
    class StateError < RuntimeError; end

    # Error raised when session misses a mapper
    class MissingMapperError < RuntimeError; end

  end # Session
end # ROM

require 'rom/session'
require 'rom/session/reader'
require 'rom/session/state'
require 'rom/session/operand'
require 'rom/session/registry'
```

Add rom-relation and rom-mapper deps
user
```ruby
require 'backports'
require 'adamantium'
require 'equalizer'
require 'abstract_type'
require 'concord'

require 'rom-relation'
require 'rom-mapper'

# Namespace module
module ROM

  # Session namespace
  class Session

    # Error raised on illegal state
    class StateError < RuntimeError; end

    # Error raised when session misses a mapper
    class MissingMapperError < RuntimeError; end

  end # Session
end # ROM

require 'rom/session'
require 'rom/session/reader'
require 'rom/session/state'
require 'rom/session/operand'
require 'rom/session/registry'
```
assistant