In the mechanics of systems involving dry frictional contacts it is known that the problem of evaluating at some instant the contact forces and the accelerations may admit a plurality of solutions. Starting from an elementary example of wedging one proposes, as a consistent way of handling information, to include the contact forces in the description of each state. This in fact is what popular time-stepping computation techniques do, as they make reference at each time-step to the constraint state calculated in the antecedent one (actually a way of treating Coulomb law incrementally). The Contact Dynamics technique optionally offers the alternative of discarding this information, exploring the consequent indeterminations and displaying the sets of solutions as clouds of dots. Examples arising from Granular Mechanics are presented. The so-called Painlev´e paradox is commented. Finally, an explanation of the isostaticity of the equilibrium of a collection of frictionless round grains is proposed.