  The main result of this paper is that the polytope of the bipartite TSP is
significantly different from that of the general TSP. Comb inequalities are
known as facet defining ones in the general case. In the bipartite case,
however, many of them are satisfied whenever all degree and subtour elimination
constraints are satisfied, {\em i.e.}\ these comb inequalities are not facet
defining. The inequalities in question belong to the cases where vertices of
one of the two classes occur in less than the half of the intersections of the
teeth and the hand. Such side conditions are necessary, as simple example shows
that the comb inequality can be violated when each class has vertices in more
than the half of the intersections.
