Comment #8 on issue 2020 by ellisonbg: Symbols & Matrices http://code.google.com/p/sympy/issues/detail?id=2020
I agree that adding shape attribute to Symbol or Expr is a bad idea. We need to get away from adding logic to the core to support new subclasses of Expr. What we really need are custom Add, Mul, Pow, etc. classes that know how to handle these things. This is now possible with the new _op_priority attribute of Expr. Also, to really do this right, we need to add the notion of a mutable Basic as well as a richer and extensible logic for combining arguments in __new__ (something more like Mathematica's rules, patterns and transformations).
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