  We study the phase diagram of a two-dimensional assembly of bosons
interacting via a soft core repulsive pair potential of varying strength, and
compare it to that of the equivalent system in which particles are regarded as
distinguishable. We show that quantum-mechanical exchanges stabilize a "cluster
crystal" phase in a wider region of parameter space than predicted by
calculations in which exchanges are neglected. This physical effect is
diametrically opposite to that which takes place in hard core Bose systems such
as $^4$He, wherein exchanges strengthen the fluid phase. It is underlain in the
cluster crystal phase of soft core bosons by the free energy gain associated to
the formation of local Bose-Einstein condensates.
