>Also, it doesn't seem to be particularly useful unless you have "objects" ... so there are some language dependencies. Using "scoping rules", I can affect polymorphism in a procedural language, but it's pretty clumsy.

That depends on what you define as "clumsy". I used to do this in 2.6 all the time:

SET PROCEDURE TO pig
DO speak

SET PROCEDURE TO cow
DO speak


*pig.prg
PROCEDURE speak
?"Oink"

*cow.prg
PROCEDURE speak
?"Moo"