  The calculus of looping sequences is a formalism for describing the evolution
of biological systems by means of term rewriting rules. We enrich this calculus
with a type discipline to guarantee the soundness of reduction rules with
respect to some biological properties deriving from the requirement of certain
elements, and the repellency of others. As an example, we model a toy system
where the repellency of a certain element is captured by our type system and
forbids another element to exit a compartment.
