  Arrow contraction applied to a tropical diagram of probability spaces is a
modification of the diagram, replacing one of the morphisms by an isomorphims,
while preserving other parts of the diagram. It is related to the rate regions
introduced by Ahlswede and K\"orner. In a companion article we use arrow
contraction to derive information about the shape of the entropic cone. Arrow
expansion is the inverse operation to the arrow contraction.
