New syntax with release 4
The Kappa syntax will change with the release 4 of KaSim (currently in pre-release). Other than cosmetics, the two major changes concern an explicit mention of the unbound (free) state of a site and a more explicit mapping between agents on the left of a rule to agents on the right (thus replacing the old "longest prefix" convention). By making the meaning of syntax less implicit than it used to be, rookie errors should be easier to avoid and Kappa rules easier to read. The new syntax has been extensively debated on the Kappa user group. Please see the manual or the changes file on github for more information. For a taste, see the example in the UI.
In addition to the usual chemistry-inspired "arrow" notation for rules (LHS → RHS), there is an "edit" notation in which a rule is expressed directly in terms of the changes it causes.