<leader> u

https://www.douban.com/group/topic/11097413/

http://www.newsmth.net/nForum/#!article/VIM/36134

http://blog.csdn.net/xiongzhengxiang/article/details/38852419

leader相当于是一个通用的命令符，默认好像是“\”，你可以在vimrc中将他改为任意一个按键，例如改为逗号：
let mapleader = ","
let g:mapleader = ","

https://www.cnblogs.com/wangkongming/p/4462272.html
http://blog.csdn.net/zcube/article/details/42298419

安装spf13的时候，.vimrc这个文件中已经有

" Key (re)Mappings {
   272 
   273     " The default leader is '\', but many people prefer ',' as it's in a standard
   274     " location. To override this behavior and set it back to '\' (or any other
   275     " character) add the following to your .vimrc.before.local file:
   276     "   let g:spf13_leader='\'
   277     if !exists('g:spf13_leader')
   278         let mapleader = ','                                                                                    
   279     else
   280         let mapleader=g:spf13_leader
   281     endif
   282     if !exists('g:spf13_localleader')
   283         let maplocalleader = '_'
   284     else
   285         let maplocalleader=g:spf13_localleader
   286     endif

另外，.vimrc中"表示注释，类似于shell中的#