As pointed out by "Paul Kulchenko" in his last sentence, I ended up adding two idioms...
local a = {}local b = {}local c,d = (a~=nil) and 1 or 0, (a~=nil) and 1 or 0 -- prints "1 1"local c,d = (a==nil) and 1 or 0, (a==nil) and 1 or 0 -- prints "0 0"print(c,d)