mettables and the io library

["D Burgess" <db@...>]

I am trying to get my head around the Lua 5 io library and 
I have questions;
In the code below how is it that the __index table recurses to
itself? Why is this done?

and, how does _input and _output make it into the metatable?

> h=io.open("x","w")
> x=getmetatable(h)
> =x
table: 00302770
> table.foreach(x,print)
lines   function: 00302320
write   function: 00302370
close   function: 003021B0
flush   function: 003023F0
__gc    function: 00302190
read    function: 00302430
_output file (00303C50)
_input  file (00303CB0)
seek    function: 003023B0
__index table: 00302770
__tostring      function: 003022D0

> table.foreach(x.__index,print)
lines   function: 00302320
write   function: 00302370
close   function: 003021B0
flush   function: 003023F0
__gc    function: 00302190
read    function: 00302430
_output file (00303C50)
_input  file (00303CB0)
seek    function: 003023B0
__index table: 00302770
__tostring      function: 003022D0