Roman-Arabic numeral converter

Today script is simple but efficient, is a Roman to Arabic (our common) numeral, it converts in both directions:

Roman 10
== "X"
>> Roman 115
== "CXV"
>> Roman 418
== "CDXVIII"
>> Roman "XIV"
== 14
>> Roman "XXIV"
== 24
>> Roman "IX"
== 9
>> Roman "LXXXI"
== 81
>> Roman "MCMLII"
== 1952
>> Roman "MMI"
== 2001
>> Roman "MMMCMXCIX"
== 3999
>> Roman 9
== "IX"
>> Roman 81
== "LXXXI"
>> Roman 1952
== "MCMLII"
>> Roman 2001
== "MMI"
>> Roman 3999
== "MMMCMXCIX"


Here is the source code:

REBOL [
    Title: "Roman"
    Date: 3-Jul-2002
    Name: 'Roman
    Version: 1.0.0
    File: %roman.r
    Author: "Andrew Martin"
    Purpose: "Converts a Roman numeral to Arabic and reverse!"
    eMail: Al.Bri@xtra.co.nz
    Web: http://valley.150m.com
    Acknowledgements: {Christian "CHE" Ensel}
]
Roman: function [
    {Converts a Roman numeral to Arabic and reverse!
    Returns 'none if it can't convert the number.}
    [catch]
    Number [string! integer!] {The Roman "MMMCMXCIX" or Arabic 3999 number to convert.}
    ][Roman-Arabic Result ][
    throw-on-error [
        Roman-Arabic: compose [
            M   1000
            CM   (-100 + 1000)
            D   500
            CD   (-100 + 500)
            C   100
            XC   (-10 + 100)
            L   50
            XL   (-10 + 50)
            X   10
            IX   (-1 + 10)
            V   5
            IV   (-1 + 5)
            I   1
            ]
        either string? Number [
            Result: 0
            if not parse/all Number [
                0 3 ["M" (Result: Result + Roman-Arabic/M)]
                opt [
                    "CM" (Result: Result + Roman-Arabic/CM)
                    | "D" (Result: Result + Roman-Arabic/D)
                    | "CD" (Result: Result + Roman-Arabic/CD)
                    ]
                0 3 ["C" (Result: Result + Roman-Arabic/C)]
                opt [
                    "XC" (Result: Result + Roman-Arabic/XC)
                    | "L" (Result: Result + Roman-Arabic/L)
                    | "XL" (Result: Result + Roman-Arabic/XL)
                    ]
                0 3 ["X" (Result: Result + Roman-Arabic/X)]
                opt [
                    "IX" (Result: Result + Roman-Arabic/IX)
                    | "V" (Result: Result + Roman-Arabic/V)
                    | "IV" (Result: Result + Roman-Arabic/IV)
                    ]
                0 3 ["I" (Result: Result + Roman-Arabic/I)]
                end
                ][Result: none ]
            ][
            if all [0 <= Number Number <= 3999] [
                Result: make string! 10
                foreach [Roman Arabic] Roman-Arabic [
                    while [Arabic <= Number][
                        append Result Roman
                        Number: Number - Arabic
                        ]
                    ]
                ]
            ]
        Result
        ]
    ]