local final_res = {}

local function amb(wordsets, res)
    if #wordsets == 0 then
        table.move(res, 1, #res, #final_res + 1, final_res)
        return true
    end
    local s = ""
    local l = #res
    if l > 0 then s = res[l] end
    res:insert("")
    for wordsets[1] as word do
        res[l + 1] = word
        if l > 0 and s[-1] != res[l + 1][1] then continue end
        if amb(wordsets:slice(2), res:clone()) then return true end
    end
    return false
end

local wordsets = {
    { "the", "that", "a" },
    { "frog", "elephant", "thing" },
    { "walked", "treaded", "grows" },
    { "slowly", "quickly" }
}

if amb(wordsets, {}) then
    print(final_res:concat(" "))
else
    print("No amb found")
end