  Let $\nat^\nat$ be the semigroup of all mappings on the natural numbers
$\nat$, and let $U$ and $V$ be subsets of $\nat^\nat$. We write $U\preccurlyeq
V$ if there exists a countable subset $C$ of $\nat^\nat$ such that $U$ is
contained in the subsemigroup generated by $V$ and $C$. We give several results
about the structure of the preorder $\preccurlyeq$. In particular, we show that
a certain statement about this preorder is equivalent to the Continuum
Hypothesis.
  The preorder $\preccurlyeq$ is analogous to one introduced by Bergman and
Shelah on subgroups of the symmetric group on $\nat$. The results in this paper
suggest that the preorder on subsemigroups of $\nat^\nat$ is much more
complicated than that on subgroups of the symmetric group.
