  A trisp closure map is a special map on the vertices of a trisp T with the
property that T collapses onto the subtrisp induced by the image of the map. We
study the interaction between trisp closure maps and group operations on the
trisp, and give conditions such that the quotient map is again a trisp closure
map. Special attention is on the case that the trisp is the nerve of an acyclic
category, and the relationship between trisp closure maps and closure operators
on posets is studied.
