Determine if sv == &PL_sv_yes

[Sisyphus]

Hi,
How does one set the value from perl-space and from C-space (ie - 2 
separate questions) so that 'sv==&PL_sv_yes' returns true ?

I've been testing for that condition as regards the third argument that 
my 'use overload' xsubs receive - and it has been working flawlessly, 
Sometimes the condition is true, sometimes not. The condition has always 
tested as I want and expect, and the response has always been 
appropriate to the condition. But all my attempts to write code such 
that 'sv==&PL_sv_yes' returns true have failed.

Here is what I tried:
use warnings;
use Inline C => Config =>
    BUILD_NOISY => 1;
use Inline C => <<'EOC';
void set_TRUE(SV * a) {
 sv_setsv(a, &PL_sv_yes);
 if(a == &PL_sv_yes)
  printf("Set to &PL_sv_yes\n"); // Never happens
}
void is_TRUE(SV * a) {
 if(SvTRUE(a)) printf("true : ");
 else printf("NOT true : ");
 if(a == &PL_sv_yes) printf("true\n");
 else printf("NOT true\n");
}
EOC
$x = '';
set_TRUE($x);
print $x, "\n";
for(TRUE, $x, 1, '1', '', 0, '0', undef) {is_TRUE($_)}
__END__
prints:
1
true : NOT true
true : NOT true
true : NOT true
NOT true : NOT true
NOT true : NOT true
NOT true : NOT true
NOT true : NOT true
I could perhaps use SvTRUE() to test that "third argument", but for the 
moment what's really puzzling me is how my current method of testing 
ever worked to begin with.

Maybe the answer would be apparent if I wrote an overloaded package and 
then did a Devel::Peek::Dump() of the third argument within the overload 
function. (I'll do that if necessary.)

Cheers,
Rob