Hint to Part 8 Exercise

Using a global command can cut the length of the command sequence
roughly in half. The correct way to use it depends on something I
did not explicitly say about global commands, but which you should
be able to guess from what I did say.

Answer to Part 8 Exercise

A fairly simple way to handle both writing the tab-revised version
of your file and keeping the original version in the editor buffer
is this sequence:
:.g/^/%s/\({*\)^I^I/\1{/g|%s/^\({*\)^I/\1    /|%s/{/^I/g|w
u

The first line is pretty straightforward, excepting the initial
global command. Otherwise it just replaces every pair of tabs at
the start of a line with the dummy character {'', then changes
any remaining solitary tab in the initial whitespace with four
space characters, changes every dummy {'' to a single tab, and
finally writes the file.

That initial global command seems silly, I know. It scans over just
the current line, it marks that line without fail because every
line has a starting point, and so it ends up running the remaining
commands on the line for sure and exactly once. This is just what
the command line would do without that initial global. So why is it
there?

The answer is in that second line. When you run an undo after a
global command, you don't just undo the last command the global
ran; you undo every buffer change done by every command the global
ran. (Note that the u is not preceded by a colon (:''); it is a
screen-mode command.) So as soon as the write is finished, the undo
puts the entire buffer back as it was.