Definition union' (s1 s2: tree) : tree :=
match compare_height s1 s1 s2 s2 with
| Lt => fold' _ insert s1 s2
| Gt => fold' _ insert s2 s1
| Eq => linear_union s1 s2
end.

Lemma diff'_spec : forall s s' x,
valid s -> valid s' -> (interp (diff' s s') x <-> interp s x /\ ~ interp s' x).