Let Σ = {0, 1, …, σ – 1}. We call Σ an alphabet and its elements are called letters; the number of letters in Σ is σ. (Except for this numerical use of σ, the “numerical” value of the letters is ignored; they are just “meaningless” characters. We use the numerals just because they are convenient characters.) A finite sequence a1a2…al, where ai is a letter, is called a word whose length is l. We denote the length of a word w by l(w). A set of (nonempty and distinct) words is called a code. For example, the code {102, 21,00} consists of three code-words: one code-word of length 3 and two code-words of length 2; the alphabet is {0, 1,2} and consists of three letters. Such an alphabet is called “ternary”.