  We study sorting machines consisting of a stack and a pop stack in series,
with or without a queue between them. While there are, a priori, four such
machines, only two are essentially different: a pop stack followed directly by
a stack, and a pop stack followed by a queue and then by a stack. In the former
case, we obtain complete answers for the basis and enumeration of the sortable
permutations. In the latter case, we present several conjectures.
