Help: PRE Actions on a wildcard (engine 4.2.20, #449)

Help/markdown/PRE.md

@@ -14,6 +14,35 @@ The PRE Action applies to each rule element in the <num, num> range individually
 
 PRE Actions operate only on tokens, or leaf nodes of the parse tree, at present. So, if the current node covers more than a *single* leaf node, the PRE Action automatically *fails*. Another thing to be careful of is that PRE Actions disregard the **base** attribute of a node. That is, even if the current node or a subnode has the **base** attribute set, the PRE Action will look under it for the leaf node.
 
+## PRE Actions on a Wildcard
+
+A PRE Action on a plain **_xWILD** element (one with no match or fail list) must hold on every node the wildcard covers, including whitespace. The wildcard is matched greedily, and it stops at whichever comes first:
+
+- the first node where a PRE Action fails;
+- the first node that matches the next rule element.
+
+The first stopping point is how a line can be gathered up to its newline:
+
+```
+@PRE
+<2,2> varz("NL");
+@RULES
+_LINE <- _xSTART _xWILD [plus] @@
+```
+
+The second lets a condition hold on everything between two elements:
+
+```
+@PRE
+<2,2> varz("num");
+@RULES
+_span <- one _xWILD [plus] three @@
+```
+
+This matches `one two three` when neither `two` nor the spaces around it has a `num` attribute.
+
+**NEW 4.2.20**: before this version, a wildcard with a PRE Action did not stop at the next rule element. It ran past it, so a rule like the second example never matched.
+
 ## See Also
 
 [Grammar Region](NLP_PP_Stuff/Grammar_Region.md)