+
+-- | Try to vectorise a top-level binding.
+-- If it doesn't vectorise then return it unharmed.
+--
+-- For example, for the binding
+--
+-- @
+-- foo :: Int -> Int
+-- foo = \x -> x + x
+-- @
+--
+-- we get
+-- @
+-- foo :: Int -> Int
+-- foo = \x -> vfoo $: x
+--
+-- v_foo :: Closure void vfoo lfoo
+-- v_foo = closure vfoo lfoo void
+--
+-- vfoo :: Void -> Int -> Int
+-- vfoo = ...
+--
+-- lfoo :: PData Void -> PData Int -> PData Int
+-- lfoo = ...
+-- @
+--
+-- @vfoo@ is the "vectorised", or scalar, version that does the same as the original
+-- function foo, but takes an explicit environment.
+--
+-- @lfoo@ is the "lifted" version that works on arrays.
+--
+-- @v_foo@ combines both of these into a `Closure` that also contains the
+-- environment.
+--
+-- The original binding @foo@ is rewritten to call the vectorised version
+-- present in the closure.
+--