Structural typing

[gvanrossum]

I'm not entirely sure what this is (probably a different work for duck typing, where two classes are equivalent if they define the same methods, public attributes etc.). In Jeremy Siek's notes:

Retroactive Conformance

Jeremy wants to allow concrete types to be usable with
abstract types for which they did not inherit from.
Structural types allow this, and there is research on how to
do this with nominal types.
Jukka voiced the concern that some of those approaches give
up modular type checking. Jeremy agreed that modular type checking
is important, but that perhaps there's a way to have our cake
and eat it too.

This issue is related to the use of the register method on ABC's to
get the right behavior with isinstance.

Structural vs. Nominal Types

We briefly discussed the possibility of having structual
object types in addition to the nominal types currently in MyPy.
Reticulated currently suppose structural object types and
not nominal types.

[JukkaL]

See the comment I added to issue #11 for a possible approach for supporting structural subtyping.

[gvanrossum]

This feels like a duplicate for #11.