The identity for alternation (the 0 element) is the empty set and identity for concatenation (the 1 element) is the set containing just the null string.

Applications

Audio and telephony

In programming for telephony, concatenation is used to provide dynamic audio feedback to a user. For example, in a "time of day" speaking clock, concatenation is used to give the correct time by playing the appropriate recordings concatenated together. For example:

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