Dice roller generator

Hello, I found this script: http://www.rebol.org/view-script.r?script=dice.r
and I supposed that it was interesting creating a dice roller generator for role-playing games like D&D, so I rewrote it and adding a GUI:

Here the source:
Rebol [
    Title: "Dice roller generator"
    File: %dice-roller.r
    Author: "Massimiliano Vessi"
    Date: 2010-12-31
    Version: 1.0.0
    email: maxint@tiscali.it
    Purpose: {From an idea of Adrew Martin a dice rolling generator for
        role-playing game like D&d or D20.}
    ]
random/seed now
roll: func [formula [string!] /local temp temp2 result test ] [
    ;test if good formula:
    test: parse formula [integer! "d" integer!]
    either test [
        temp:   parse formula "d"
        temp2:   reduce [(to-integer temp/1)   (to-integer temp/2)]
        result: 0
        loop temp2/1 [result:   result + (random temp2/2)     ]      
        result]["Invalid formula" ]
    ]  
view/title layout [
    vtext {Type your formula for dice roll (like 2d6) and push "Roll" button.}
    across
    vlab "Formula:"
    aa: field    
    btn "Roll" [
        insert bb/text ( reform [aa/text ":^/" tab (roll aa/text) newline ] )
        show bb
        ]
    return
    bb: area    
    ] "Dice roller generator"