Demo source backup

[Deleplace]

Some online playgrounds tend to come and go. When they are taken down, we end up with broken Demo links.

Currently

For each broken Demo link, the demo source is lost (not recoverable). The idiom implementation snippet does contain the core lines of code to be showcased, but not the program boilerplate that makes it immediately compilable/runnable, nor the sample values, nor the sample stdout lines.

Desired

• Try to retrieve the full demo source of all of the Demo links that are currently not broken.
• When a link goes down, try to show a read-only version of the full demo source instead