以下是一段LUA的代码，请教高手“perm{"a", "b", "c"}”为什么能返回一个table
function permgen (a, n)
 if n == 0 then
  coroutine.yield(a)
 else
  for i=1,n do

   -- put i-th element as the last one
   a[n], a[i] = a[i], a[n]

   -- generate all permutations of the other elements
   permgen(a, n - 1)

   -- restore i-th element
   a[n], a[i] = a[i], a[n]

  end
 end
end
function perm (a)
 local n = table.getn(a)
 local co = coroutine.create(function () permgen(a, n) end)
 return function () -- iterator
  local code, res = coroutine.resume(co)
  return res
 end
end

function printResult (a)
 for i,v in ipairs(a) do
  io.write(v, " ")
 end
 io.write("\n")
end

for p in perm{"a", "b", "c"} do
 printResult(p)
end
请教高手“perm{"a", "b", "c"}”为什么能返回一个table