The switch can be made on
        1. the variable "s"
        2. the read operation "getchar()"

    There is no mechanism for rewind of the input,
    no look ahead and no way to store the input internally.

    The program halts when getchar returns EOF.
    There are two possibilities
        1. exit(YES);
        2. exit(NO);

    What can we do with such a language?

    Consider the following  *UNIX operation

          ls  p[0-9]*.gif

    The patterns allowed are called regular expressions

    The simple model that we have just defined can recognize
    such expressions.

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