What we do...

alpha1.lua

--this is what we generally do

local Alpha = {}
Alpha.__index = Alpha
Alpha.alpha = true

--many lines of code later:
--to test a node's type:

if node.alpha then
    --do something
end

alpha2.lua

--this is what we could do

local Alpha = {}
Alpha.__index = Alpha
Alpha.alpha = true

--many lines of code later:
--to test a node's type:

if node.__index==Alpha then
    --do something
end

alpha3.lua

--this is what we should do

local Alpha = {}
Alpha.__index = Alpha
Alpha.isAlpha = true

--many lines of code later:
--to test a node's type:

if node.isAlpha then
    --do something
end