(autoload 'try-code "try-code"  nil t)
(global-set-key (kbd "C-?") 'try-code)

Scenario: you want to try an alternative implementation of a function while keeping the original within reach.

1. Select lines.
2. C-?
3. (optional) C-y (yank original implementation)
4. make changes as required
5. press C-? again

When you reinvoke the function, you can choose which implementation to keep.

Try it out; bug reports are welcome.