Alphabets and Sets of Strings
An alphabet ? = {a1, a2, ..., an} is a set of characters.
A string over ? is a sequence of zero or more elements of ?.
Example. If ? = {0, 1, 2} then 2201 is a string over ?.
No matter what ? is, the empty string ? is a string over ?.
A set of strings over ? is a set of zero or more strings, each of which is a string over ?.
Example. If ? = {0, 1, 2} then {?, 111, 121, 0} is a set of strings over ?.