He says that sets are like dictionaries without values, e.g. a set is akin to the dictionary keys.

So, in your example the ('a', 1) and ('a', 2) are the keys (in how the parent argues about) -- it's not 'a' that's the key.

Same like you can do today:

  d = {}
  d[('a', 1)] = 6
  d[('a', 2)] = 89

  {(('a', 1), 6), (('a', 2), 89)}

> dicts would subclass sets

Was trying to make the case as to why treating dictionaries as a child of <generic collection> with an extend/merge operation using ‘+’ rather than a child of set with a union operation ‘|’ makes more sense (to me).

Changing the behaviour of a well defined operation like union seems bad - although my case is somewhat undermined by python’s overloading of ‘+’ to mean extend.