via (https://www.linux.com/learn/tutorials/442438-vim-tips-folding-fun)

- `zf#j` creates a fold from the cursor down # lines.
- `zf/string` creates a fold from the cursor to string .
- `zj` moves the cursor to the next fold.
- `zk` moves the cursor to the previous fold.
- `zo` opens a fold at the cursor.
- `zO` opens all folds at the cursor.
- `zm` increases the foldlevel by one.
- `zM` closes all open folds.
- `zr` decreases the foldlevel by one.
- `zR` decreases the foldlevel to zero -- all folds will be open.
- `zd` deletes the fold at the cursor.
- `zE` deletes all folds.
- `[z` move to start of open fold.
- `]z` move to end of open fold.