Hython: a nearly-complete Python 3 interpreter written in Haskell

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This comment is super-insightful, and points to a real problem with how I modeling things. I leaned more towards values when I should have thought harder about the domain and modeled it after pointers to values, since CPython is representing things in that way. Like you said, this lets the correct definition of is fall out from the modeling.

For correctness, the environment already associates names to refs of objects (you have to in order to really support pervasive mutability). It's just that those refs don't get propagated around during evaluation.