This function can be used to transpose stretches of text:
This function exchanges two nonoverlapping portions of the buffer (if they overlap, the function signals an error). Arguments start1 and end1 specify the bounds of one portion and arguments start2 and end2 specify the bounds of the other portion.
Normally, transpose-regions relocates markers with the transposed
text; a marker previously positioned within one of the two transposed
portions moves along with that portion, thus remaining between the same
two characters in their new position. However, if leave-markers
is non-nil, transpose-regions does not do this—it leaves
all markers unrelocated.