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.