Re: why (error "Transposed region has length 0") ?

[Richard Stallman]

The story of transpose-regions is peculiar.  It was meant to be used
in transpose-subr, but we never put it into service there because
nobody was ever convinced that it really worked correctly in all
cases (it is rather a complex thing to check) and nobody felt like debugging 
whatever problems it might have.

The function is used in just one Emacs package, hanoi.el.  I geuss
that has given it a certain amount of testing, but not very broadly.

The main motivation for transpose-regions was to reduce memory usage,
sinceit avoids copying both regions into strings.  Nowadays that
optimization may not be worth the trouble.  We could simply delete the
current transpose-region and use 6 lines of Lisp code (see
transpose-subr-1) instead.