Conditional execution of query
This function evaluates a condition. Note that the values used in the
else clause are evaluated lazily. They may be expressions
that involve stored queries (i.e. queries stored using the
keyword). These queries will not be evaluated if they are not needed.
This allows a query to cheaply branch. For example, if a parameter is
given, then perform hash or upload to the server. See the
Windows.Search.FileFinder for an example of how
if() is used.