@@ -195,22 +195,45 @@ value. However, if ``A`` is not set, the variable is set to "aval".
Setting a weak default value (??=)
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-It is possible to use a "weaker" assignment than in the previous section
-by using the "??=" operator. This assignment behaves identical to "?="
-except that the assignment is made at the end of the parsing process
-rather than immediately. Consequently, when multiple "??=" assignments
-exist, the last one is used. Also, any "=" or "?=" assignment will
-override the value set with "??=". Here is an example::
-
- A ??= "somevalue"
- A ??= "someothervalue"
-
-If ``A`` is set before the above statements are
-parsed, the variable retains its value. If ``A`` is not set, the
-variable is set to "someothervalue".
-
-Again, this assignment is a "lazy" or "weak" assignment because it does
-not occur until the end of the parsing process.
+The weak default value of a variable is the value which that variable
+will expand to if no value has been assigned to it via any of the other
+assignment operators. The "??=" operator takes effect immediately, replacing
+any previously defined weak default value. Here is an example::
+
+ W ??= "x"
+ A := "${W}" # Immediate variable expansion
+ W ??= "y"
+ B := "${W}" # Immediate variable expansion
+ W ??= "z"
+ C = "${W}"
+ W ?= "i"
+
+After parsing we will have::
+
+ A = "x"
+ B = "y"
+ C = "i"
+ W = "i"
+
+Appending and prepending non-override style will not substitute the weak
+default value, which means that after parsing::
+
+ W ??= "x"
+ W += "y"
+
+we will have::
+
+ W = " y"
+
+On the other hand, override-style appends/prepends/removes are applied after
+any active weak default value has been substituted::
+
+ W ??= "x"
+ W:append = "y"
+
+After parsing we will have::
+
+ W = "xy"
Immediate variable expansion (:=)
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