Proof. (i) First, we prove that for all terms t ∈ Wτ (X) and for all
B ⊆ Wτ (X), if xj 6∈ V ar({t}), then there exists a term t
0 ∈ Wτ (X) such
that
{t} ·xi B = {t
0
} ·xj B.
If xi 6∈ V ar({t}), then {t} ·xi B = {t} = {t} ·xj B. Assume that xi ∈
V ar({t}). Then we proceed by induction on the complexity of term t. If
t = xi
, then with t
0 = xj we have {t} ·xi B = B = {t
0} ·xj B. Let t =
fi(t1, . . . , tni
). Then from xj 6∈ V ar({t}) there follows that xj 6∈ V ar({tk})
for all 1 ≤ k ≤ ni and inductively we assume that there are terms t
0
k
such
that {tk} ·xi B = {t
0
k
} ·xj B for all 1 ≤ k ≤ ni
. Then
{t} ·xi B