Question regarding coroutines

[Diman Todorov <diman@...>]

Hello list,

narf = function()
        a = coroutine.yield()
        coroutine.yield()
        print(a)
end

co1 = coroutine.create(narf)
co2 = coroutine.create(narf)

coroutine.resume(co1)
coroutine.resume(co2)

coroutine.resume(co1, "narf")
coroutine.resume(co2, "zod")

coroutine.resume(co1)
coroutine.resume(co2)

<snip>
Quintillian:~/tmp diman$ lua narf.lua
zod
zod
Quintillian:~/tmp diman$
<snip>

It seems, that LUA uses the same memory to store the variable 'a' for  
both coroutines
Is this a feature, is it a bug and how can i avoid it?
Can i not somehow 'duplicate' the function prior to resuming it?

regards
Diman