Definition: Let S be an alphabet.
1) e is a string over S
2) if x is a string over S and a Î S then xa is a
    string over S
Example: Consider S = {0, 1}:
Is string over S.
:
:
...
e
0
1
 =  0
 =  1
1
Note: e denotes the empty string that contains no symbols.
e
e
0
0
1
1
0
0
1
1
10
10
0
S
String
 Gist: x = a1a2…an
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