Pyrsistent is a number of persistent collections (sometimes referred
 to as functional data structures). Persistent in the sense that they
 are immutable.
 .
 All methods on a data structure that would normally mutate it instead
 return a new copy of the structure containing the requested updates.
 The original structure is left untouched.
 .
 The collection types and key features currently implemented are:
 .
  * PVector, similar to a python list
  * PMap, similar to dict
  * PSet, similar to set
  * PRecord, a PMap on steroids with fixed fields, optional type and
    invariant checking and much more
  * PClass, a Python class fixed fields, optional type and invariant
    checking and much more
  * Checked collections, PVector, PMap and PSet with optional type
    and invariance checks and more
  * PBag, similar to collections.Counter
  * PList, a classic singly linked list
  * PDeque, similar to collections.deque
  * Immutable object type (immutable) built on the named tuple
  * freeze and thaw functions to convert between pythons standard
    collections and pyrsistent collections.
  * Flexible transformations of arbitrarily complex structures built
    from PMaps and PVectors.