The author thanks the referee for his helpful suggestions concerning the presentation of this paper.
We are grateful to the referee for a number of helpful suggestions for improvement in the article.
At the suggestion of the referee, we consider some simple cases.
Go to the list of words starting with: a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
t
u
v
w
y
z