Consider the code

$//$ PRE: x$>=0,$ y$>=0,$ both integers

$//$ POST: True

function f$($x$)$:

while $!($x$==0)$ or $!($y$==0)$:

if $!($x$==0)$:

x $=$ x $-1$

else:

x $=30$

y $=$ y $-1$

return True