about the Pth whitepaper...

hello,

i carefully read the original whitepaper but there were
a couple of lines of the model implementation whose
meaning eludes me!

i know there is a mailing list specifically for
Pth but i am loitering here on account of another matter
anyway (previous post), so ...

my question relates to step 7 and the restoration of
the old 'alternative' stack:

sigaltstack(NULL,&ss);
ss.ss_flags = SS_DISABLE;
sigaltstack(&ss,NULL);
if(! (oss.ss_flags & SS_DISABLE) )
   sigaltstack(&oss,NULL);

firstly, it retrieves the current alt-stack, disables
it then re-instates it. but why is it of any use to keep
around the stack description of another thread?
surely it has served its purpose and can be
immediately replaced with oss.

secondly, oss is only re-instated if not disabled.
but why make this check? if it was disabled before
entry to mctx_create what harm is there in leaving it
this way on return? why leave in place the new thread's
stack details (disabled) if oss was itself disabled?

any insight appreciated!
- chris