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Simon Peyton Jones authored
The new simplifer stuff exposed the fact that the invariants on the TvSubstEnv and IdSubstEnv were insufficiently explicit. (Resulted in a bug found by Sam Brosnon.) This patch fixes the bug, and tries to document the invariants pretty thoroughly. See Note [Extending the TvSubst] in Type Note [Extenting the Subst] in CoreSubst (Most of the new lines are comments.)
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