Ltac dsepcon :=
match goal with
  [ H : sepcon _ _ _ |- _ ] =>
  let s1 := fresh "s" in
    let s2 := fresh "s" in
      let JOIN := fresh "JOIN" in
        let H1 := fresh "H" in
          let H2 := fresh "H" in
      destruct H as [s1 [s2 [JOIN [H1 H2]]]]
end.

Lemma nn_njoin sigma x z s1 s2 : next_in_dom x sigma = true -> space_denote sigma s1 -> space_atom_denote (Next (Var x) z) s2 -> joins s1 s2 -> False.