let constant fmt = function
| Fol.ConstInt n ->
if n.[0] = '-'
then Format.fprintf fmt "(%s)" n
else pp_print_string fmt n
| Fol.ConstBool b -> pp_print_string fmt (if b then "true" else "false")
| Fol.ConstUnit -> pp_print_string fmt "void"
| Fol.ConstFloat f ->
if f.[0] = '-'
then Format.fprintf fmt "(%s)" f
else pp_print_string fmt f