+However, note that any keys not listed in an alist definition will
+still be overwritten; they will be reset to zero (except
+@code{stretchability}, which takes the value of @code{space}).
+Thus the following two declarations are equivalent:
+
+@example
+system-system-spacing =
+ #'((space . 10))
+
+system-system-spacing =
+ #'((padding . 0)
+ (space . 10)
+ (minimum-distance . 0)
+ (stretchability . 10))
+@end example
+
+One possibly unintended consequence of the above example is the
+removal of the default values for @code{padding} and
+@code{minimum-distance}. Defining a variable as an alist (of any
+size) will always reset all its default key-values. Default
+settings for the flexible vertical @code{\paper} dimensions are
+defined in @file{ly/paper-defaults-init.ly}.
+