Lemma 6 An ordered semigroup S is left strongly simple if and only if
a ∈ (SbSa] for every a, b ∈ S.
Proof. =⇒. Let a, b ∈ S. Since S is simple, by Lemma 1, we have (SbS] = S,
then a ∈ (SbS]. Since S is left quasi-regular, we have a ∈ (SaSa]. Thus we
get a ∈ (SaSa] ⊆ (S(SbS]Sa] = (S(SbS)Sa] ⊆ (SbSa].
⇐=. If a ∈ S, by hypothesis, we have a ∈ (SaSa], so S is left quasi-regular.
Let now a, b ∈ S. By hypothesis, we have a ∈ (SbSa] ⊆ (SbS(SbSa]] =
(SbS(SbSa)] ⊆ (SbS], and S is simple.