* Make the _replace() method respect subclassing.

* Using property() to make _fields read-only wasn't a good idea.
  It caused len(Point._fields) to fail.

* Add note to _cast() about length checking and alternative with the star-operator.

With some changes of my own thrown in (e.g. backport of r58107).

(suprisingly, this simplifies the signature, improves clarity, and is comparably fast).
Update the docs to reflect a previous change to the function name.
Add an example to the docs showing how to override the default __repr__ method.

Let the field spec be either a string or a non-string sequence (suggested by Martin Blais with use cases).
Improve the error message in the case of a SyntaxError (caused by a duplicate field name).

Add maxlen support to deque() and fixup docs.
Partially fix __reduce__().  The None as a third arg was no longer supported.
Still needs work on __reduce__() to handle recursive inputs.

Allows optional commas in the field-name spec (help when named tuples are used in conjuction with sql queries).
Adds the __fields__ attribute for introspection and to support conversion to dictionary form.
Adds a  __replace__() method similar to str.replace() but using a named field as a target.
Clean-up spelling and presentation in doc-strings.