sorteddict [was a PEP proposal, but isn't anymore!]

[Terry Reedy]

"Duncan Booth" <duncan.booth at invalid.invalid> wrote in message 
news:Xns99BC8BF9F1899duncanbooth at 127.0.0.1...
| Subclassing doesn't have to imply a common implementation, just a common
| interface.

True, but in Python, subclassing is usually done to reuse implementation. 
Interface subclassing is usually from a common abstract base class (which 
will be better supported in Py3)..