Ralf I added the following to user_guide.txt. I tested the examples using my tracker
with link (for examples that make sense) and multilink properties.

Bern, is there anything you think should be added here?

Do both of you think this looks good enough to close the documentation requirement
for this issue?

We still need a populated db and tests for this codepath.

Advanced Searching with Property Expressions
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can create more advanced searches in your tracker by using Reverse
Polish Notation (`RPN`_) to make property expressions. By default, when
filtering by multiple items, the expression type is 'or'. For
instance, if you filter the property assignedto by adding the query
string element "assignedto=2,3,40", it matches users "2 or 3 or
40". In RPN, this would be written as "2, 3, or, 4, or". Roundup uses
negative numbers to represent operators. For example, using "-1" for a
single value (e.g. the assignedto Link property, but not the keyword
multivalued/MultiLink property) matches an issue where the property is
'not set'.

The operators and their corresponding numbers are:

* 'not' is represented by -2
* 'and' is represented by -3
* 'or' is represented by -4

So, "assignedto=2,3,40" is the same as
"assignedto=2,3,-4,40,-4". While this example is the same as
"2,3,40", the expression "keyword=1,2,-3,-2" filters issues that don't
have both keywords 1 and 2.

Another example is: ``creator=3,-2,1,-2,-3``. This is the same as the
expression: ``(not user3) and (not user1)``. Using the rules of logic,
this is the same as: ``not (user3 or user1)`` which is expressed in
RPN as ``creator=3,1,-4,-2``. Compare this to ``creator=3,1,-2`` which
returns issues created by user3 or any user other than user1.

Typing these expressions manually can be tiresome, so there's an
expression editor on the search page. You can access it by clicking on
the ``(expr)`` link, which makes creating these expressions a bit
easier.