Proof. Assume that A = B·xiA. There follows xi ∈ V ar(B) since
otherwise, i.e. if xi 6∈ V ar(B), we have A = B·xiA = B and this contradicts
xi ∈ V ar(A). This means, B0 6= ∅. Suppose xi 6∈ B. Then op(b) ≥ 1 for
all b ∈ B0
. By xi ∈ V ar(A) we have also A0 6= ∅. Let s be the least natural
number such that A0
s 6= ∅. Consider h ∈ A0
. Because of A = B ·xi A and
B = B0 ∪ B00 we get