vim中let与set如何区分，一直挺困惑，上网搜索了一翻，把结果记录于此。
set设置的是选项， let设置的是变量。
:set is for setting options, :let for assigning a value to a variable.
It happens that the value for an option is linked to the name of the option prepended by a & (the &option-name construct then behaves very similar to “ordinary” variables). So, the following are equivalent:
:set tw=40
:let &tw=40
But, for example, assigning 50 to the global variable foo (:let g:foo=50) cannot be achieved with a :set command (because g:foo is a variable and not an option).
Some options are boolean like. When setting these, no value is needed (as in :set noic and the opposite :set ic).