+Note [Shadowing]
+~~~~~~~~~~~~~~~~
+In this pass we gather up usage information that may mention variables
+that are bound between the usage site and the definition site; or (more
+seriously) may be bound to something different at the definition site.
+For example:
+
+ f x = letrec g y v = let x = ...
+ in ...(g (a,b) x)...
+
+Since 'x' is in scope at the call site, we may make a rewrite rule that
+looks like
+ RULE forall a,b. g (a,b) x = ...
+But this rule will never match, because it's really a different 'x' at
+the call site -- and that difference will be manifest by the time the
+simplifier gets to it. [A worry: the simplifier doesn't *guarantee*
+no-shadowing, so perhaps it may not be distinct?]
+
+Anyway, the rule isn't actually wrong, it's just not useful. One possibility
+is to run deShadowBinds before running SpecConstr, but instead we run the
+simplifier. That gives the simplest possible program for SpecConstr to
+chew on; and it virtually guarantees no shadowing.
+
+