Roman numerals. Values greater than or equal to 1 are valid roman
numerals.
val of_roman : string -> t Core.Std.Or_error.t
Parse string as a roman numeral if possible. Case-insensitive.
val of_arabic : int -> t Core.Std.Or_error.t
Convert integer to roman numeral. Return Error if given int is
less than 1.
val to_roman : t -> string
String representation of a roman numeral.
val to_arabic : t -> int
Integer representation of a roman numeral.