Lemma 2.1. Assume that an infinite sequence of letters of a finite alphabet
B = {B1,...,Bn} is not ultimately periodic. Then, for every M ∈ N, there
is a block X of length M and two different letters Bi, Bj ∈ B such that the
sequence contains infinitely many blocks of the form BiX and BjX. Similarly,
there is a block Y of length M and two different letters Bu, Bv ∈ B such that
the sequence contains infinitely many blocks of the form Y Bu and Y Bv.