We assume that Alice and Bob communicate by sending sequences of symbols from a finite set *Σ*, which we call the **alphabet**. We always use *q* to stand for the size of the set of symbols, |*Σ*|. A **word** is a sequence of symbols from the alphabet *Σ*. If *w<sub>1</sub>w<sub>2</sub>...w<sub>n</sub>* is such a word, then *n* is its **length**. We use *Σ<sup>n</sup>* to denote the set of words with length *n* using symbols in *Σ*. In general, the number of words in *Σ<sup>n</sup>* is
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