String#+@ and String#-@
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.
+'foo'returns modifiable string.
-'foo'returns frozen string (because wasters will freeze below 0 degree in Celsius).