The '''empty string''' is a string containing no characters.  Because of Tcl's
[copy-on-write] implementation of values, there is precisely one empty string. In a Tcl script, there are exactly three notations for the empty string:
`{}`, `""`, and `[[some_command_that_returns_the_empty_string]]`.

[AMG]: Actually, you can obtain empty string by typing `[[]]`.

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