  Let $x$ and $y$ be words. We consider the languages whose words $z$ are those
for which the numbers of occurrences of $x$ and $y$, as subwords of $z$, are
the same (resp., the number of $x$'s is less than the number of $y$'s, resp.,
is less than or equal). We give a necessary and sufficient condition on $x$ and
$y$ for these languages to be regular, and we show how to check this condition
efficiently.
