Allow detaching bits of the tree / comment out code / copy-n-paste about primer HOT 1 OPEN

While playing with https://studio.code.org, I discovered that they have a clever way of commenting out code by wrapping code in an "unused code" block:

You can move an "Unused code" block into another block and it will automatically be uncommented. Here I dragged the previously unused code block "make a new sprite called mySprite with costume foo" into the main program, at which point it automatically became active code again:

When new code blocks are initially dragged into the canvas but are not initially attached to other blocks, or when code blocks are dragged out of another block and placed on their own, they are not initially wrapped in "Unused code" blocks. But once you run the program, any unattached blocks are wrapped in an "Unused code" block.

There's also a "comment" block that you can attach to other blocks to add a comment that doesn't affect the program's behavior.

from primer.