Macro argument quoting and ifelse condtional

[Bill McCarty]

Hi all,

I'm new to m4 and finding an apparently common idiom
confusing. Can someone help me understand it?

Assuming quotes have been replaced by << and >>, a
typical macro definition of the sort that puzzles me
reads as follows:

define(<<_CF_OUTPUT>>, <<dnl
ifelse(_CF_$1, <<_CF_$1>>, <<>>, <<
divert
CF_$1_OUTPUT
divert(-1)
>>)>>)dnl
dnl

I'm particularly puzzled by the distinction between
the first two arguments of ifelse: _CF_$1 and
<<_CF_$1>>. The ifelse seems to cause the macro to
expand to void unless its argument is double-quouted,
like <<<<TEST>>>>. But, in actual use contexts, its
argument seems generally to be single-quoted. So,
genius that I am <g>, I conclude that I'm missing
something. But, I can't see what.

Thanks for any help!