FoldingByMark

Keys: create:`zf` close:`zc` open:`zo` sync:`zx` open-all:`zR` close-all:`zM`

Folding is when
```
{
  hello
  world
}
```
is turned into
```
+{}
```
or something. In graphical editors we see things
like boxed plusses and lines indicating the scope
of foldings. But vim is restricted to the terminal.

## Links
* [Vim from scratch article](https://www.vimfromscratch.com/articles/vim-folding)