Lemma 3.6. Let L and L′ be two GADLs with 0, 0′, respectively. If f : L → L′
is a homomorphism, then for any non-empty subset A of L, we have
f(A∗) ⊆ {f(A)}∗.
Proof. Let x ∈ f(A∗) and y ∈ f(A). Then there exists a ∈ A∗ and b ∈ A such
that x = f(a) and y = f(b). Now x∧y = f(a)∧f(b) = f(a∧b) = f(0) (∵ a ∈ A∗
and b ∈ A)= 0′. That is x∧y = 0′ for all y ∈ f(A). Hence x ∈ {f(A)}∗. Therefore
f(A∗) ⊆ {f(A)}∗.
If L is a GADL with 0, then for any A ⊆ L, {f(A)}∗ = f(A∗) is not true in
general. Consider the following example.