  We provide a characterisation of strong bisimilarity in a fragment of CCS
that contains only prefix, parallel composition, synchronisation and a limited
form of replication. The characterisation is not an axiomatisation, but is
instead presented as a rewriting system. We discuss how our method allows us to
derive a new congruence result in the $\pi$-calculus: congruence holds in the
sub-calculus that does not include restriction nor sum, and features a limited
form of replication. We have not formalised the latter result in all details.
