From feace8d26b1ed573abbef4118c70ce4f2d9c1d23 Mon Sep 17 00:00:00 2001
From: Tobias Beer <beertobias@gmail.com>
Date: Tue, 12 Dec 2017 14:46:12 +0100
Subject: [PATCH] added gotcha examples to Filter Expressions and rephrased a
 little for simplicity (#3055)

---
 .../filters/syntax/Filter Expression.tid      | 32 +++++++++++++++----
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/editions/tw5.com/tiddlers/filters/syntax/Filter Expression.tid b/editions/tw5.com/tiddlers/filters/syntax/Filter Expression.tid
index 694f44dc2..430a900cd 100644
--- a/editions/tw5.com/tiddlers/filters/syntax/Filter Expression.tid	
+++ b/editions/tw5.com/tiddlers/filters/syntax/Filter Expression.tid	
@@ -1,5 +1,5 @@
 created: 20150124182421000
-modified: 20150124184900000
+modified: 20171212073225103
 tags: [[Filter Syntax]]
 title: Filter Expression
 type: text/vnd.tiddlywiki
@@ -14,15 +14,35 @@ type: text/vnd.tiddlywiki
 
 A <<.def "filter expression">> is the outermost level of the [[filter syntax|Filter Syntax]]. It consists of one or more [[runs|Filter Run]].
 
-A run's input is normally a list of all the non-[[shadow|ShadowTiddlers]] tiddler titles in the wiki (in no particular order). But the `+` prefix can change this.
+If a run has:
 
-* If a run has no `+` or `-` prefix, its output titles are [[dominantly appended|Dominant Append]] to the filter's output.
-* If a run has a `-` prefix, its output titles are <<.em removed>> from the filter's output (if they were present).
-* If a run has a `+` prefix, it receives the filter's output so far as its input. The filter's output is then completely <<.em replaced>> by the run's output. Any subsequent run reverts to receiving all tiddler titles as its input, unless it too has a `+` prefix.
+* no prefix, its output titles are [[dominantly appended|Dominant Append]] to the filter's output
+* the prefix `-`, output titles are <<.em removed>> from the filter's output (if such tiddlers exist)
+* the prefix `+`, it receives the filter output so far as its input; its output then <<.em "replaces">> all filter ouput so far and forms the input for the next run
 
-In concise technical terms:
+In technical / logical terms:
 
 |!Run |!Interpretation |!Output |
 |`run` |union of sets |... OR run |
 |`+run` |intersection of sets |... AND run |
 |`-run` |difference of sets |... AND NOT run |
+
+A run's input is normally a list of all the non-[[shadow|ShadowTiddlers]] tiddler titles in the wiki (in no particular order). But the `+` prefix can change this:
+
+|Prefix|Input|h
+|`-` or none| <$link to="all Operator">`[all[]]`</$link> tiddler titles, unless otherwise determined by the first [[filter operator|Filter Operators]]|
+|`+`|the filter output of all previous runs so far|
+
+Precisely because of varying inputs, be aware that both prefixes `-` and `+` do not behave inverse to one another!
+
+For example, in both of the following, `$:/baz` will only be removed if it actually exists:
+
+* <$link to="is Operator"> `foo bar $:/baz -[is[system]]`</$link>
+* <$link to="prefix Operator">`foo bar $:/baz -[prefix[$:/]]`</$link>
+
+To understand why, consider the input for both final runs with their `-` prefix.
+
+In order to remove `$:/baz` in any case, existing or not, simply use the `+` prefix with [[negated filter operators|Filter Operators]]:
+
+* <$link to="is Operator">`foo bar $:/baz +[!is[system]]`</$link>
+* <$link to="prefix Operator">`foo bar $:/baz +[!prefix[$:/]]`</$link>