grXML grammar question

Hi,

I'd like a grammar able to return 3 spelled letters to my application like :

   <grammar root="command">
        <rule id="command" scope="public">
            <item>
             <ruleref uri="#firstletter"/>
             <ruleref uri="#secondletter"/>
             <ruleref uri="#thirdletter"/>
            </item>
        </rule>

        <rule id="firstletter">
             <item><ruleref uri="#alphabet"/><tag>firstletter=$alphabet</tag></item>
        </rule>

        <rule id="secondletter">
             <item><ruleref uri="#alphabet"/><tag>secondletter=$alphabet</tag></item>
        </rule>

        <rule id="thirdletter">
             <item><ruleref uri="#alphabet"/><tag>thirdletter=$alphabet</tag></item>
        </rule>

        <rule id="alphabet">
             <one-of>
                <item>a<tag>alphabet="a"</tag></item>
                <item>b<tag>alphabet="b"</tag></item>
                <item>c<tag>alphabet="c"</tag></item>
                <item>d<tag>alphabet="d"</tag></item>
                <item>e<tag>alphabet="e"</tag></item>
                <item>f<tag>alphabet="f"</tag></item>
                <item>g<tag>alphabet="g"</tag></item>
                <item>h<tag>alphabet="h"</tag></item>
                <item>i<tag>alphabet="i"</tag></item>
                <item>j<tag>alphabet="j"</tag></item>
                <item>k<tag>alphabet="k"</tag></item>
                <item>l<tag>alphabet="l"</tag></item>
                <item>m<tag>alphabet="m"</tag></item>
                <item>n<tag>alphabet="n"</tag></item>
                <item>o<tag>alphabet="o"</tag></item>
                <item>p<tag>alphabet="p"</tag></item>
                <item>q<tag>alphabet="q"</tag></item>
                <item>r<tag>alphabet="r"</tag></item>
                <item>s<tag>alphabet="s"</tag></item>
                <item>t<tag>alphabet="t"</tag></item>
                <item>u<tag>alphabet="u"</tag></item>
                <item>v<tag>alphabet="v"</tag></item>
                <item>w<tag>alphabet="w"</tag></item>
                <item>x<tag>alphabet="x"</tag></item>
                <item>y<tag>alphabet="y"</tag></item>
                <item>z<tag>alphabet="z"</tag></item>
               </one-of>
          </rule>
   </grammar>

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The problem is i don't know how to take the value returned by the "alphabet" rule into my
"firstletter", "secondletter" and "thirdletter" rules.

if the user say "a....b.....c" i have to store "abc" into a variable.
How could i do ? Maybe it is possible to make like :

<rule id="letters">
    <item repeat="3"><ruleref uri="#alphabet"/><tag>xxxxxxxxx</tag></item>
</rule>

but what code as to be placed instead of the "xxxxxxxx" to make (result returned by alphabet) + (result returned by alphabet) + (result returned by alphabet) -> String




Thanks for your help

    Yahn Teisseire