type pos = {file : string; line : int; column : int}
type region = {left : pos; right : pos}
type 'a phrase = {at : region; it : 'a}
val no_pos : pos
val no_region : region
val string_of_pos : pos -> string
val string_of_region : region -> string
val (@@) : 'a -> region -> 'a phrase
val at : region -> 'a -> 'a phrase