allow arg on one line

Author: EmileTrotignon

lib/Params.ml

@@ -221,7 +221,8 @@ module Exp = struct
     kw_out_of_box
     $ box_decl
         ( kw_in_box
-        $ hvbox_if should_box_args 0 (args $ fmt_opt annot $ fmt_opt epi) )
+        $ hvbox_if ~name:"should_box_args" should_box_args 0
+            (hvbox_if (not (ocp c)) 0 args $ fmt_opt annot $ fmt_opt epi) )
 
   let box_fun_expr (c : Conf.t) ~source ~ctx0 ~ctx =
     match ctx0 with