+ -- The (VarEnv Var) is just a VarSet. But we write it like
+ -- this to remind ourselves that you can look up a Var in
+ -- the InScopeSet. Typically the InScopeSet contains the
+ -- canonical version of the variable (e.g. with an informative
+ -- unfolding), so this lookup is useful.
+ --
+ -- INVARIANT: the VarEnv maps (the Unique of) a variable to
+ -- a variable with the same Uniqua. (This was not
+ -- the case in the past, when we had a grevious hack
+ -- mapping var1 to var2.
+ --
+ -- The FastInt is a kind of hash-value used by uniqAway