zipper verses originate from back in the days when there was only one
song.

later....

union organizers and protest leaders used them to great advantage
(woody guthrie did this) by using tunes that people were familiar
with and adding lyrics that spoke about the topic du jour. (sp?)

example....Red Wing (a song about an indian maiden) became Union Maid

other lyrics were "zipped in"
they could have called them applique' lyrics
or velcro
or glue stick...