  A model is presented consisting of triangular trimers on the triangular
lattice. In analogy to the dimer problem, these particles cover the lattice
completely without overlap. The model has a honeycomb structure of hexagonal
cells separated by rigid domain walls. The transfer matrix can be diagonalised
by a Bethe Ansatz with two types of particles. This leads two an exact
expression for the entropy on a two-dimensional subset of the parameter space.
