Fix CL for diamond formula sets with all agents

For the case of two (or more) diamond formulas who all mention the full
agent list, no rule was created, because the former CL algorithm only
created rules for proper subsets of the agent list. This adds the
missing rule and lets cool correctly answer the following query:

    $ ./coalg.native sat CL --agents '1' <<< '(~[{1}] p) & ~[{1}] ~p'
    unsatisfiable