+Note [Float coercions]
+~~~~~~~~~~~~~~~~~~~~~~
+When we find the binding
+ x = e `cast` co
+we'd like to transform it to
+ x' = e
+ x = x `cast` co -- A trivial binding
+There's a chance that e will be a constructor application or function, or something
+like that, so moving the coerion to the usage site may well cancel the coersions
+and lead to further optimisation. Example:
+
+ data family T a :: *
+ data instance T Int = T Int
+
+ foo :: Int -> Int -> Int
+ foo m n = ...
+ where
+ x = T m
+ go 0 = 0
+ go n = case x of { T m -> go (n-m) }
+ -- This case should optimise
+