-> -- Function of elt of input list
-> -- and accumulator, returning new
-> -- accumulator and elt of result list
-> -> accl -- Initial accumulator from left
-> -> accr -- Initial accumulator from right
-> -> [x] -- Input list
-> -> (accl, accr, [y]) -- Final accumulator and result list
+> -- Function of elt of input list
+> -- and accumulator, returning new
+> -- accumulator and elt of result list
+> -> accl -- Initial accumulator from left
+> -> accr -- Initial accumulator from right
+> -> [x] -- Input list
+> -> (accl, accr, [y]) -- Final accumulator and result list