  Finite smooth digraphs, that is, finite directed graphs without sources and
sinks, can be partially ordered via pp-constructability. We give a complete
description of this poset and, in particular, we prove that it is a
distributive lattice. Moreover, we show that in order to separate two smooth
digraphs in our poset it suffices to show that the polymorphism clone of one of
the digraphs satisfies a prime cyclic loop condition that is not satisfied by
the polymorphism clone of the other. Furthermore, we prove that the poset of
cyclic loop ordered by their strength for clones is a distributive lattice,
too.
