A string is a finite sequence of symbols from 
.
An alphabet is a finite, nonempty set of symbols. We use
to denote this alphabet. Note: Symbols may be more than one English letter
long, e.g. while is a single symbol in Pascal.
A string is a finite sequence of symbols from .
The length of a string s, denoted |s|, is the number of symbols in it.
The empty string is the string of length zero.
It really looks like this:
but for convenience we usually write it like this: 
denotes the set of all sequences of strings that are composed of zero or more
symbols of .
denotes the set of all sequences of strings composed of one or more symbols
of . That is,
=
- {}.
A language is a subset of .