Using variables in classes definition

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I need to define a variable and a class depending on that variable. Something 
like :

<example>
control:
        servers = ( host1 host2 )

classes:
        i_am_a_server = ( ${servers} )
</example>
It doesn't work because :
- ${servers} must be enclosed in quotes for syntax needs
- if ${servers} is enclosed in quotes, it is considered as a command+parameters 
which must be executed and whose result is used to define i_am_a_server.

I need both the variable and the class because somewhere later in my script I 
give the variable value as a parameter to an external script (shellcommands), 
and some sections of the script are activated only if the running host belongs 
to the list "servers".
Of course I can repeat the hosts list on the "i_am_a_server" class definition, 
but each time I modify "servers" I must then think to modify also 
"i_am_a_server" :-(

I couldn't find something to do that in the docs. Does somebody has any idea ?

Patrice

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