Ltac landL :=
    repeat match goal with
    | |- ?L1 //\\ ?L2 |-- ?R =>
        match L1 with
        | context [R] => apply landL1
        | _ =>
          match L2 with
          | context [R] => apply landL2
          end
        end
    end.

Lemma landC P Q: P //\\ Q -|- Q //\\ P.