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-- make global variables readonly
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local f=function (t,i) error("cannot redefine global variable `"..i.."'",2) end
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local g={}
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local G=getfenv()
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setmetatable(g,{__index=G,__newindex=f})
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setfenv(1,g)
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-- an example
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rawset(g,"x",3)
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x=2
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y=1 -- cannot redefine `y'
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