+"""Module for query operations
+
+How it works:
+
+ - Add field definitions
+ - See how L{NODE_FIELDS} is built
+ - Each field gets:
+ - Query field definition (L{objects.QueryFieldDefinition}, use
+ L{_MakeField} for creating), containing:
+ - Name, must be lowercase and match L{FIELD_NAME_RE}
+ - Title for tables, must not contain whitespace and match
+ L{TITLE_RE}
+ - Value data type, e.g. L{constants.QFT_NUMBER}
+ - Data request type, see e.g. C{NQ_*}
+ - A retrieval function, see L{Query.__init__} for description
+ - Pass list of fields through L{_PrepareFieldList} for preparation and
+ checks
+ - Instantiate L{Query} with prepared field list definition and selected fields
+ - Call L{Query.RequestedData} to determine what data to collect/compute
+ - Call L{Query.Query} or L{Query.OldStyleQuery} with collected data and use
+ result
+ - Data container must support iteration using C{__iter__}
+ - Items are passed to retrieval functions and can have any format
+ - Call L{Query.GetFields} to get list of definitions for selected fields
+
+@attention: Retrieval functions must be idempotent. They can be called multiple
+ times, in any order and any number of times. This is important to keep in
+ mind for implementing filters in the future.
+
+"""