Increment numbers in Vim

How to increment numbers in VIM with a quick trick?
101 This is an item.
101 This is an item.
102 This is an item.
103 This is an item.
104 This is an item.
and so on

There are several ways to make a list of increasing numbers. One simple method is to use Ctrl-A in a macro. As an example, suppose you type the line:
In normal mode, enter the following to record a macro into the a register (type the characters shown, without pressing Enter). This macro yanks the current line, then pastes it below, then increments the number.
Now type 15@a to perform the macro 15 times.

You will see:

Later, you might insert a new item after #102.
Now you need to renumber the following items (the new item will be 103, so the old 103 has to be incremented, as does 104, and so on.
That can be done using :.,$g/^\d/exe "normal! \<C-a>"