Issue #11782 has been reported by Koichi Sasada.

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Feature #11782: String#+@ and String#-@
https://bugs.ruby-lang.org/issues/11782

* Author: Koichi Sasada
* Status: Open
* Priority: Normal
* Assignee: Yukihiro Matsumoto
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Matz said 

> In fact, my best choice is introducing `String#+` that returns a mutable copy of a string.
[ruby-core:71879] [Ruby trunk - Bug #11759] 

So that this is a ticket for that.
I'll commit it ASAP to check this methods before 2.3.

Specification:

* `+'foo'` returns modifiable string.
* `-'foo'` returns frozen string (because wasters will freeze below 0 degree in Celsius).




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