let pretty fmt = function
| Here -> Format.pp_print_string fmt "\\here"
| Pre -> Format.pp_print_string fmt "\\pre"
| Post -> Format.pp_print_string fmt "\\post"
| Exit -> Format.pp_print_string fmt "\\exit"
| LabelParam label -> Format.fprintf fmt "Label '%s'" label
| CallAt sid -> Format.fprintf fmt "Call sid:%d" sid
| At(label::_,_) -> Format.fprintf fmt "Stmt '%s'" label
| At([],sid) -> Format.fprintf fmt "Stmt sid:%d" sid