Confused about proper tail calls

["Tai Meng" <tmeng@...>]

Hi, I'm trying to understand how proper tail calls work in conjunction with the Lua debug interface. 

I'm having difficulties understanding this paragraph from section 2.5.7 of the Lua 5.0 user manual: 

> Lua implements proper tail calls
> (or proper tail recursion): In a tail call, the called function reuses the stack entry of the calling
> function. Therefore, there is no limit on the number of nested tail calls that a program can execute.
> However, a tail call erases any debug information about the calling function.

I originally understood this as: when function A makes a proper tail call to function B, the stack containing A's activation record is *replaced* by that of B (since A's stack entry is reused). However, this does not seem to be true. When I ran a script containing a recursive function, this is the call stack I got:

========= CALL STACK ============

Function 0: myth() @../../scripts/factorial.lua, line: 3
Function 1: made tail call so its info is erased by Lua.
Function 2: made tail call so its info is erased by Lua.
Function 3: made tail call so its info is erased by Lua.
Function 4: made tail call so its info is erased by Lua.
Function 5: made tail call so its info is erased by Lua.
Function 6: main() @../../scripts/factorial.lua, line: 7

===== END CALL STACK ============

So, main() -- the main chunk -- called myth(), which called itself several times. I had expected only 1 stack for myth(). But having not observed that, I placed a conditional in the code, so that if the activation record retrieved by lua_getstack() had its attribute 'what' equal to 'tail', I output the message seen beside functions 1 to 5.

My call stack code is simplified below, and the Lua script is further below. Am I doing something wrong? Or is this a known functionality of Lua?

Thanks a lot,

Tai

---- call stack display code ------

void DisplayCallStack( lua_State* L, lua_Debug* ar )
{
    while ( lua_getstack( L, level, ar ) == 1 ) // 1 = success
    {
        if ( lua_getinfo( L, "Sln", ar ) != 0 ) // no errors
        { 
            // print out info about current stack.
            // if ar->what == 'tail', output tail call message
        }
        level++;
    }
    lua_getstack( L, 0, ar ); 
    // return to the state before this function call.
}

---- myth.lua ------------------

function myth( n )
    if n == 0 then return 0 end
    return myth(n-1)
end

s = myth(5)