+# to improve robustness, and you come up with a nice implementation
+# that doesn't require extra parentheses in the user's LIST.
+#
+# But wait - now the algorithm is quadratic, because every recursion of
+# the algorithm keeps the entire LIST and merely adds another m4_shift to
+# the quoted text. If the user has a lot of elements in LIST, you can
+# bring the system to its knees with the memory m4 then requires, or trip
+# the m4 --nesting-limit recursion factor. The only way to avoid
+# quadratic growth is ensure m4_shift is expanded prior to the recursion.
+# Hence the design below.
+#
+# The M4 manual now includes a chapter devoted to this issue, with
+# the lessons learned from m4sugar.